Should my friend be blaming me for him cheating on his own girlfriend?

While it is his fault, you should have let him speak up for it. Or you could have just been like, "I don't know, you should take that up with him". I'm friends with a few guys who are in a band and they have girlfriends and if there ever came a time where their girlfriends asked me if they were cheating on them, or wanted dirt about them, I wouldn't tell them anything. The simple reason being I am friends with them and not their girlfriends. I'm not about to rat them out no matter how bad it is. My loyalty would go to my friend's, not whoever the person is they are dating. As bad as it sounds, it's life.

No, it not your fault, but than sometimes, these subjects are better off just listening it and let it go. You dont want to get caught in middle of it. I been there and dont want to do it again. But you friend need to realized that he is the one who cheated on his ex gf and dont need to blame you for his exgf finding out about it.

It's his fault; he was, and is, wrong. You were just responding truthfully. Why would it matter if they had already broken up?