That is not cool. Tell your bff how it made you feel when she went behind your back to your boyfriend like that. On the positive side it's great that your boyfriend told you what was going on instead of trying to hook up with her on the sly. That's a guy you can trust. He's a keeper.
At your friend's age, and yours, people get new feelings about things every 20 minutes or so. Relax. This is normal. You are all friends. Laugh it off and get along with everyone. Your girlfriend will turn her feelings in another direction soon and everything will be fine.
first you should tell him your going to confront your friend in infront of him, because for all we know he could be testing you to see how you really feel about him, see what his response is. And if he's telling the truth, confront your friend in front of your boyfriend, she'll probably denie it but, after what ever happens, I would not trust her alone with your boyfriend. good luck, keep me posted.
I understand why your mad if she was a true friend i dont think she would go around telling your boyfriend something like that there are plenty other guys for her to talk to instead of trying to hit on your boy thats wrong. no friend should do that then they arent a real friend