Hi TiFF, welcome to the forum
She feels cheated on by the both of you. Not only her boy-friend. You would feel the same, I think, if this would happen to you.
My best advise is not to go on seeing her ex-boyfriend since that will hurt her even more. This, of course, if you care for your best-friend. It can be that she will never 'forgive' you (and him) but I think the most ethical thing to do is to steer clear of messing up this mess even more.