What do I do about my best friend’s boyfriend?

Nothing, you don't do anything WITH HIM. Stay away from him, unless you only do FRIENDLY things together, otherwise you'll ruin their relationship AND lose your friend.Edit: Also, how would you feel dating a person who CHEATED to be with you?

Guys are not worth destroying friendships over in this situation.

Like the answers above. That guy might seem like the most important thing to you but you'll probably become a jealous girlfriend if you ever go out with him because of what he did with you he might do with another girl. If he cheated on your best friend he can cheat on you. I don't think you want to be with a cheater?? You'll also lose your best friend in which are hard to find now a days. My advice is probable not what you want to hear but you should really think about it! Best of luck

You know the saying sisters before misters? FOLLOW it!!! Dont have anything to do with the guy... You should value your friendship with this girl more than whatever is between you and the guy. You risk loosing both of them! Besides you dont want to be with a guy who would cheat on his girlfriend, especially with someone she trusts. Tell the guy that what he is doing is unnacceptable and if he continues this behaviour, you will tell your friend. Both you and your friend deserve guys who you can trust and respect... Good luck! You have a tough situation here...

just tell her the truth if she hears it from anyone else she will never forgive you but if youre the one who tells her you can clearify what happened exactly and she wont be all caught up in those little lies that people add onyou wont be able to keep boh because either way one of them will get hurt