5 ideas on what to do if you worry that a person close to you is suffering in a toxic relationship
Worrying is a sign of care and empathy, but also of projecting our own story of fears into the lives of others that may or may not be in line with their situation. Even if it is, they may be absolutely fine with it because they have different standards (= wants and desires => lessons left to learn) from ours. That said,
Realize that everybody now is exactly where they have projected themselves to be; they are 100% responsible and have 100% ownership of their case - you do not have a share in it, and they do not have a share in yours! You are both free-will creatures, and giving each other this unconditional freedom is the utmost form of respect, so forget about "white" lies, "noble" manipulations, and "the-end-justifies-the-means" strategies when it comes to you trying to help a fellow - strategizing this way is dehumanizing and depriving you of your human dignity. Keep your motivation clean and in check; intervening in someone's most intimate business is a delicate thing you really don't want to mess up! Your relationship with this person is at stake every time you challenge the half of them reserved for their partner because it signals you do not like and approve of them, which is the exact opposite of what you truly feel for them if you have dedicated of your time to provide some help.
Even if you have managed to point your close person's attention to what you consider to be an unwise wanting on their part (=to be engaged in a toxic relationship) and they absolutely agree with your analysis of their situation, at the cost of some or all of their pride (which is a very high price to charge them for your advice and support; much more expensive and much less reliable than a professional psychologist!), recognize how hard you hit, and how deep you cut in your attempt to help. You are putting your (hopefully clean) fingers in their wounds, and then you should get them out so they can recover. Meaning you have to leave them alone and let them process your intervention. It will take time until some positive effect of your touch (=words and actions) is visible. Meanwhile, don't take it personally if they react hostile to your "first aid" because pain is ... well, painful. Patients come to professionals not out of the enthusiastic desire to live (if they were really keen on living, they wouldn't get sick) but out of the fear of death (which is the same as the fear of life). They are never happy at the operation table or when the needle needs to get in. When your close person has trusted you to perform a sort of operation on their soul, you should enter the venture with your entire awareness and full responsibility. So be patient - they will either embrace or reject your advice according to the level of their readiness and progress in their self-healing process. Either way, you should be O.K. with whatever they decide to do next!
By being patient, I mostly mean not repeating the same message (words and/or actions) twice so they listen attentively every time. Repetitions wear out your messages and become tedious and annoying side noise they stop caring about. Suppose you get compulsive in your desire to help. In that case, this is not a sign of "motherly" care but a signal that something with your motivation or patience is wrong, and they will start ignoring or running away from you eventually. If your words and actions resonate with them, your exact words will chase them and stick around like a tattoo on their brain until the issue is solved; therefore, no repetition is needed. If your words and actions do not resonate with them, no repetition could make them do so before the situation changes.
Whatever you do, be sure that you stay honest and informative but also kind and understanding. For example, you may sound something like, "I will always love you (as a friend/ family/ the love of my life/ etc.), but I don't like the things you do right now to yourself and your current partner because of evidence 1,2,3... I believe you can make it right for the betterment of both of you by doing this or that! I hope you find the strength within you to fight your demons that got you here in the first place, which is: something, something. Go back to where your fears have fooled and misled you and made you cruel in your overuse of self-defense. I will wait for you on the other side of hate (towards yourself, your partner, and me) if you choose to get there, and I will be very sad if you don't. I will miss you, but I will not visit you in your dark place of willful helplessness. You know where to find me if you need me because you have figured out you love me too. I do not love you because I need you; I love you because it feels good when I do." Of course, your phrasing will differ from mine, but that really does not matter unless you have your message on record - then you have to be extremely careful! If you happen to advise your close person in a natural conversation, remember the general rule that a person may forget your exact words and actions and may twist them into false memory in their mind as time goes by and in accordance with what they would like to believe in at present, but they will never ever forget how you made them feel in the gut and in their heart.
If you have made them feel good in your company and you have provided them with a temporary sense of relief and consolation, you can expect them to return for more of this "sweet anesthetic." The problem with people who are really engaged in a toxic relationship, though, is that they are so engulfed in their suffering that they are not able to relate to others as friends without clinging to them. The paradox is that they are hurting so much, yet they cannot take the best pill ever (friendship) because they are incapable of it at this point. You can lend a sympathetic eаr, and a shoulder to cry on once or twice at most but do not teach them to use you as a door mat, washing machine, or battery charger. Refusing to sort their baggage for them is not unkind; it is the kindest thing you can do to yourself directly and to them indirectly. Having clear boundaries about what (even the closest of) people can and cannot use you for is everything. If you cannot stand for yourself, no one will, and you risk getting bitter and disappointed with their "ungrateful" responses. Do not lend a helping hand if you are not feeling good about it at the very moment of doing so or if you have some back-office calculations of your potential future returns on the investment. Such delayed rewards do not work - they only trap you in the fiction of never-ending unfulfillable expectations. Learn to listen to yourself in the present moment, and you will be able to provide the best help without even thinking about it. In the long run, you can be and do for others only what you can be and do for yourself. Ultimately, the best gift you can give your close person is to restore their belief in themselves that they are capable of achieving whatever they want, be that the separation from their toxic partner or not.
If you suspect that your close person is trying to tie you as a third wheel to their toxic relationship where you play the part of the "savior" in the victim-tyrant-savior triangle of abuse, cut these attempts right on the spot. It may be hard for you to overcome your tender feelings for your close person, but you absolutely must resist the temptation of being the one who is always there hanging around on a wire, putting your life at pause, waiting for them to wake up. Do not apply the I-will-be-around-when-he/she-is-gone strategy because being a rebound is not only offensive and humiliating but also damaging to your relationship because your close person starts developing an association between you and some form of an "emotional chemical toilet" for their soul shit to be discarded and decomposed by you.
If you want anyone to follow your advice, practice what you preach so that you 1) give them a good example, 2) test the feasibility of your advice first-hand, and 3) be a genuine and reliable source of wisdom instead of being a hypocrite. Then, in theory, advising from tonnes of experience would appear to be the most reliable. But in practice, it rarely works this way because masters have gone such a long way that they have already forgotten how to translate their wisdom for laymen and beginners. The better guide to a new terrain is the one who walks a few steps ahead of you, not a mile. There is also the possibility that the more you master a field, the less mysterious and thus boring it becomes, so you abandon it. (I have treated this question at length in my essay Talking Artworks.) The paradox is that those who preach the best are those who need to learn what they preach the most. The moment the curios study of the novel field is gone, the channeling downwards towards students stops too. Maybe life has presented to you the case of your close someone to learn from it simultaneously while providing help for them. This way, your help runs both ways to come back to you. Be grateful you have the opportunity to be of service; it is how you evolve as well.
Once you and your close person have both learned your lessons from the case at present - you have played the part of a teacher for them, and they have followed, doing the same for you in return, your relationship grows deeper, more equal, and balanced to the point when you would refuse to do therapy with them even if you are capable of doing it because you like each other too much to do that anymore. In general, I am a very strong supporter of professional psychological help. I believe it saves friendships, marriages, and lives, so if you do not feel capable of performing the above 5 steps, then maybe it is a better idea to only refer your close person to a professional and spend your time in each other's company having a good time up high instead of digging in their dirty laundry that is none of your business anyway.