Drunk or not, she did wrong, if you have the kind of relationship, where you KNOW that is a no no. She could have just made a mistake and learned her lesson. If your friendship is worth it, just get over it. If she is still a complete loser in your eyes....drop her like a hot potatoe. Keep in mind that your "ex" boyfriend is an ex for a reason. You just need to ask yourself if your friendship is worth it. If she does have more to hide, do you really want to know? Is an ex-boyfriend worth losing a friend over? From your question, it's not clear whether your boyfriend was still your boyfriend when your best friend messed around with him, or whether he was already an ex boyfriend when she messed around with him. If he was still your boyfriend, then what she did should be grounds to part company and find yourself another best friend...you don't need friends like that. If he was already an ex when she messed around with him, it shouldn't matter a great deal other than it would have been nice if she had told you of her intentions beforehand.