Unfortunately I've been in a similar situation where someone I was close with had been in a toxic relationship. At the end of the day their entire dynamic skyrocketed my anxiety. She never made an attempt to break it off with him, even though she did agree with me when I expressed how unhealthy the relationship was.

Like you said: there does come a time where you need to choose between your own well being and that of a friend who is an adult that has not and will not heed your advice to make their lives better.

Some extra info: I've known her and her boyfriend for 8 years (I actually met her through him in fact). Twice he broke up with her then begged for her back, each time she accepted him again. She has very little positive things to say about him and it's gotten in the way of our friendship. Whenever we want to hang out together (as in have a girl's night) he makes it an issue and takes it out on her. He also claims that I am his friend while telling her not to see me alone because I'm bisexual and that "something could happen" (even though they had been broken up before and nothing happened between me and her). He's controlling, unpleasant and emotionally unstable. If I want to see her I need to see him as well otherwise he makes it difficult for both of us. I can't think of what else to do than to end my relationship with both of them and move on.