Nothing - let him go and don't look back, you don't need that and deserve better - move on.
If you and your boyfriend are still dating and have not split up then this friend of yours is not a true friend to you and you should consider her a toxic friend and walk away from her. If you and your boyfriend broke up and she dated him and obviously didn't tell you it is probably because she didn't want to hurt you. In this case talk with her and tell her that she should have been honest with you, but you should still remain friends.
You should tell her, they should be ok with it because they are not in love with their cousin and plus if she is really your best friend it will be cool with her.
Say to your bf: " Look we have been dating. I get it shes your best friend but she should stay your best friend and I can stay your girl friend. I really like you! Please don't break my heart."
i dont have a friend that has an ex girlfriend that is dating his friend when your friend is on a summer vacation so i cant no no no no no a
if they are your best friend they should understand
If you dumped them because you can't stand them then it hardly makes sense to be their friend. They'll still have the same irritating habits as when you were dating so it's best to move on.
Not to far because if yall end up dating then most likely you will HATE eachother!
yer, why not
Talk to her about it.
no
I think you should ditch your best friend and replace her with another person who won't betray your trust by dating your boyfriend. That's what you should do.
You should talk to your friend who is dating the other friend's ex about telling her. If she refuses and continues to see the ex, you might want to evaluate her interpretation of friendship and what kind of friend she is. Based on that, you may want to tell the friend yourself if the other refuses.