Would you be able to forgive a friend for this...?

I would forgive both of them. I think both of them were in the wrong to go behind your back and have , but considering that you and the guy were on a break, that does change things quite a bit. Honestly, I think the guy is more in the wrong here than your friend. I wouldn't go out with the guy again, just stay friends. And I would forgive the girl and become friends again, maybe not best friends but at least enough to be cool. Neither of them seem very trust worthy but I don't think you have to cut them out of your life completely.