If she is indeed mentally ill, he needs to perservere and be insistent and get her real help. Try different doctors if he needs to, etc. If he's in physical danger, he could try for a restraining order, but if she's the "love of his life," he'll not want to abandon her quite so readily ...

I think staying in touch and being a friend and a nonjudgemental sounding board is the best possible thing you could do for him right now. He should probably be seeing a counselor as well, as dealing with a loved one who is mentally ill is taxing on anyone.