I've been on your friends side of the whole mess before, no fun. I was able to realise that I could like her as a friend, me her and my friend are all still friends, we hang out alot. I heard about a website,
www.myspace.com , its like a people web page, I've gotten a couple of dates off of it, been boosting my self confidence. Its been good for me. I think you should just tell him, as a couple, tell him that you two are together, hope he will still be friends with you guys. I have found, being up front, is good, because, even though it might hurt a little more, the hurt isnt as long lasting, and not as much resentment.
good luck dude!