Really messed up stuff.

Well, I guess first of all, I would have a chat with the two of them and find out how long as this been going on. Talking face to face it's the only way you're going to get some sort of closure, since they can't hide behind mysterious text messages or chats. Secondly, dump the guy. Unless you think this sort of behaviour is acceptable (hint: it isn't) than you better find someone who will treat you right and respect you like he should. Have a serious chat with your best friend alone and then have a good thought about this: Does this person deserve your friendship? Can this person be trusted anymore? Did she go too far? Would I ever be able to forgive her? And eventually you'll come to a conclusion which is either: a) forgive her, or b) never talk to her again. Focus on yourself, how you're coping, take good care of your mental and physical health notwithstanding the hurt and sadness and anger you might feel. It'll get better. You'll find people that deserve you, people better than those two.

while i agree with @whitepeachjelly that you should dump the guy, i disagree that you only have those two options with your best friend. i've been in the same situation before, and my best friend had meant a lot to me and i didn't want to lose a good friendship over one mistake. so if you don't like what your friend did, but you don't exactly want to end the friendship completely, you can just play it safe. if the both of you can go along with it, you can just not tell her anything you'd have to put any trust into (ex. who your boyfriend is) but still treat her with the same respect you guys had as close friends, and slowly let yourself heal from that experience and have her gain your trust back without losing a friend. i don't think the boyfriend is worth trying to work things out with, but the friend might be. idk man everyone makes mistakes and that's your homegirl, you know?