Usual indignation about his obvious lack of honor (not to mention propriety) aside, it sounds like he has a bit of a history of being a cad. If I had to take a guess, I would say that this was a relationship already on the rocks, and the forwarded message was just the catalyst. I'd let your friend (who forwarded your note) off the hook, she was just trying to help the girlfriend. It made a hash of things, but it was at least a little noble.
On the "do we stay friends" question, I've had female friends put in that situation before, and it never ends well. At best, he's still legitimately hung up on you (which would also explain why the girlfriend blames you for the breakup). At worst, he screwed up his relationship all on his own, and is looking not only for a rationalization, but also for some comfort. Either way, it's not a good basis for any kind of relationship.
You could try to give him closure, but at this point you should ask yourself whether you owe him even that. If you do, then it'd be worth it to sit down and try to clear the air. If you decide you don't, then you should tell him that you have no interest in resuming a romantic relationship, nor being his emotional crutch.
