My best friend's boyfriend is suicidal and psychotic.She is smart yet won't leave him. He verbally abuses her.?

He abuses her verbally all the time and is constantly undermining her confidence. She is alwasy always on the phone to him, day and night because he lives in her hometown while she is in college.He is always shouting down the phone telling her she is a ****** and stupid. Because the abuse is not physical she does not recognise just how dangerous and damaging it is. He is completely selfish and blames her for everything that goes wrong in his life. He has anger management problems and loses the cool all the time. He threats to commit suicide and tells her he wishes she could see This is nearly making her suicidal. She is a very smart girl yet simply will not leave him.Its killing me because I live with her and it hurts me so much to watch her slip from being the lovely bubbly girl I know to a depressed victim. I have woken to hear her crying or screaming down the phone on several occasions. What should I do?!