They say that when the boy you like is in amor with another girl is better to forget him!But this is different...

Yes, the boy i like is now in amor with another girl!
The problem is that this girl is not for him...
She is already has a boyfriend 1 ano and a half, she likes 4 other boys,the 3 of them are in in amor with her also and the one of them is his best friend!!!
It's not fair to lose their friendship(they are friends mais than 5 years)for a girl like her...
I don't know what to do, she was my friend before she started flirting with him now i hate her and she mad with me, but i haven't done anything to her, she has!
Anyway i think that if i don't let him go, i can help him to get over her(i don't know how)
I know it's complicating...i need someone's help!(for the most of my problems here!)