That's a tough situation to be in. First, I think you need to not beat yourself up about and not question whether telling your BF was the right thing to do or not - if that's what you felt was the right thing considering your relationship, then it was the right thing to do. Keeping a good relationship for yourself is more important than keeping secrets to preserve another person's relationship.
So my advice is don't second guess yourself for what has already happened, but look at the situation as it now stands and figure out what, if anything, you want to do about it. In the short term, it might be best to just let things lie for a little while and see how things shake out. It sounds like this relationship has been having problems for a while now. Sometimes it's hard for people to recognize that a relationship is failing and they want to blame someone else, but eventually they realize the truth. It very well might be that in a few days, weeks, or months, this couple will realize that no one else is to blame for the problems in their relationship. If they want to continue to blame you, cut your losses and move on. You and your boyfriend don't need a friendship with people who are would rather lie and backstab to cover their own mistakes than maintain an honest friendship.
I hope you feel better about things soon. It sounds like a really crummy situation to be caught in the middle of.
Mom to rescued boxers Dallas and Gracie