I had a friend that was like a brother to me. Then his boyfriend tried to sleep with me and I was cut out. I think he blames me, and people can have their opinions, but I genuinely care for this friend and would never desire to hurt him. He rejects and ignores all of my reaching out. I don't foresee us ever having a relationship again. For a while, I was angry that he'd blame me. Now I'm just nostalgic for the time we were friends and wish things could go back to the way they were. I live in the past because I'm a little lost without this friend. I've tried to replace him, but he was one of a kind.
What I know, from the experiences in my own life and in the lives of others, is that time usually (not always) softens feelings. If my hunch is correct, your former friend's boyfriend will likely pull this trick again, and again, and probably again. Before too long, your former friend, if he has any rudimentary mathematical skills, will start putting one and one together and discover that he has a cheater on his hands. You've done your part by reaching out. You must decide whether or not you will extend your friend more grace than he has given you and welcome him back into your life, if and when he comes to his senses. Friends are precious so I hope it comes to that.