I would personally say if she is that bad off, he needs to remove the hope and replace it with a call to the police and or social services that she has made threats of being a danger to herself. Sadly, I am speaking from experience. He can't save her. I had to have someone do that for me actually. I was in the other girl's position and as mad as it made me for them to do it, I did finally get some real help. When I was in college, I was not being treated for my clinical depression. I was trying to use relationships as crutches. However, no one can save you, and they couldn't save me either. Finally, my lover left under no uncertain terms. I had to have them back... I just had to... And in my depressed and untreated state, I made a few threats to my health. I just knew they would come running. Well, someone did... The police showed up and I did a 72 hour hold. It got me the help I really needed though. Sometimes, even white knights have to realize that they cannot wear the armor and save a drowning maiden.... Everyone will drown.
Let her stand on her own two feet. Tough love is still love and sometimes the best love you can give someone. If she threatens to hurt herself, do not go running. Report it to the police. They can get her the help she needs. If she decides to not get help after that, at least she knows that she needs to stand on her own feet.