I read through the thread a few times today, and mulled over my reply and finally did.
I told him that I thought he was a neat person, but that given how we'd met (i.e. not with platonic intent) I didn't think I could switch gears to pure friendship, plus I didn't think it was fair to his girlfriend and I didn't think it was the right thing to do to her or to me.
I also said I was in the area for another year and if he found himself single again, he could contact me, but only if he was single because I expected him to hold himself to fairness and honesty.
He's not a WS, he's just a guy, dating in the 40+ world which is tough and sucks. I didn't want to be a jerk about it to him and I think I accomplished that. I think if the girlfriend sees this email I comported myself well and she will see I respected both her and me. And I think that's the best I could have done today.