He may have been your friend but your GF is still to blame. Your friend isn't in that sort of relationship with you so he can do whatever he wants (even though it's wrong). Your GF obviously was unhappy with the relationship and chose to try her luck elsewhere. People don't cheat for no reason, they cheat because they're bored of their relationship or something in the relationship is missing so people go somewhere else to get what their relationship isn't giving them.
Your friend shouldn't have done it but she's still the one to blame even more for being with you and choosing to hit on someone else, let alone choosing a guy who she knows you are friends with. Guys will be guys and, even though they might know in their minds it's wrong, it can be hard for them to turn down some free vag.
It's actually too complicated to put into words properly.
But this guy still expected me to be friends with him, like he'd done nothing wrong. The way he told me was "implying", so when I said "Why the fuck did you do that?", he was saying "Well, I didn't say that, I was implying it, so therefor you don't know", I'm like "I'm not a fucking idiot".
I'm pretty over it now, but it really pissed me off to the point of turning into a dickhead for a few months to get attention, it wasn't something that happened once either, with all the other things they were doing, it could be classed as psychological bullying, if you think about it. It wasn't just a simple case of some free vag.
---------------
Anyway, back on topic, I'm now picturing that fight as you two kiddies slapping each other in the face a few times, then walking off.