When my cousin's wife was admitted to the psych ward for the third time he got some sound advice. Everytime he got close to leaving she'd make a half-hearted attempt on her own life. The doctor told him that not only was he not helping her by staying with her, but he was enabling her manipulative behavior and one of these days she might succeed by accident. He filed for a divorce that day.

Now this woman was under medical care, if the woman you speak of is not seeking care I would talk to another one of her loved ones and simply say that you cannot deal with this relationship right now and you are afraid for her safety.