Well, you're in a very sticky situation right now. I'm going to say you should probably 'take a break' from seeing your boyfriend because the stress being put on you (and your relationship itself) by this friend that you have.
Because of the sudden change in this friend's behavior (the accusation, which sounds completely false) you might feel yourself become very prone to being upset and you could snap at your boyfriend without fully realizing it.
I really think you should tell your boyfriend about how your friend used him, just so that he knows whats going on and can gain perspective on the situation. It just sounds fair to let him know whats happening - imagine how confused the poor guy must be.
And last but not at all least - talk to your friend. Tell her that her behavior is making you upset. You can use "I" statements, which I will put examples of later. "I" statements help the other person gain your perspective of thought and can help prevent resorting to actual fighting. Its also a really good idea to try and stay calm when you talk to her, but nobody will blame you if you get upset.
Here are some no-no statements:
"You're such a b***h and maneater!"
"I hate you / You've ruined my life!"
Here are the better revisions of said statements:
"I was really upset when you did that."
"I got really mad when you did that."
You can also explain to her that you don't want to lose her, because you two are friends. Friends make it through thick and thin, and you're just hitting a rocky road. But, if nothing at all works with this girl, I gotta say that you have to move on sometimes...
Best of luck, hope I helped !! [: