really want my best friend to break up with her boyfriend...?

to put this briefly...

he's her first boyfriend. don't get me wrong, he's my friend too i've heard both sides of the story..

- they've been together for about 9 months, had sex, both 15, and this relationship just seems so destructive.

- my friend has depression and jealousy issues. it's ridiculous, she won't let him do anything or talk to anyone and no matter what he does, she always finds fault and is just constantly upset because of him. she wont accept his compliments, or him telling her he loves her, and brings everyone around her down - including me and i'm her best friend - because she will just launch into depressive mode and refuse to accept help. i'm not being a b*tch here, we've all tried to help her so much, i've stayed up so many nights all night talking to her and trying to make her see sense but nothing gets through and it's just like she's self destructing. this relationship is NOT good for her.

- he on the other hand, flirts with numerous other girls including me who is going out with his bestfriend, and claims to be 'bi' and wants to experiment with other guys and it makes my friend feel unloved. he is very inappropriate in how he speaks to girls, and he ignores her for long periods of time and i just don't know why this relationship is still continuing.

i've left out a lot but this is just the bare, bare outline of it all... advice please?