My friends gf is a but he won't believe me what should I do?

Okay so I have a friend and I have feelings for him and right now he's internet dating some chick named hayley and they have been together 4 2 mnths I think. Anyway I have a crush on my friend but everyone knows that this girl is a but everytime I try to tell him that he says"I'm changing her" and I tell him"you can't change people" and he says"yes you can' ugggh it's so frustrating sometimes I think I'm just jealous though. But seriously though this girl is a everyone knows it she loves and she can't go a day without it she even admitted it and I have a bad feeling shes not staying truthful to him bc the first time they started dating her mom didn't like it and threatend to put the law on him then they had to break up now they are back together again and I don't trust that girl one bit and also it so obvious she only wants him 4 but he can't see that bc he's infatuated with her and she claims she"loves him" but its all about lust I mean what does a 17 yr old girl want with a 15 yr old boy she's just taking advatage of him and I'm scared 4 him bc when he gets depressed he gets crazy one time he tried to commit suicide and he cuts and I I don't want to see him get hurt, so what should I do? plz olz tell me