The unwritten rules of friendship. Never have a relationship with someone your best friend has feelings for. Never sleep with your best friend's exes, no matter what they say. Even if they say it's okay and they approve, don't do it if you are a true friend. And if you do, you're not a true friend.
Stephanie had strong feelings for this guy she once pretended to hate, yet her best friend completely overlooked this by disobeying the best friend code and having a relationship with him. Now they are no longer friends.