If your signifigant other was cheating on you with your best friend, would you like to know about it so that you could decide if that is the type of relationship you would like to continue with, so you actually have a choice? Or would you rather live by ignorance is bliss? I would rather have a friend that could be honest with me even in tough times, even if the truth might hurt, then have myself aligned with people who do not respect me, lie to me, hurt me and don't value me enough to let me decide what to do with the truth that involves my life in a very direct mannor. Taking away someone choice through lying by ommision is manipulitive. Your friend will be hurt either way...because his gf doesn't sound like she respects him either

I think you are right to tell your friend. But do it because he deserves better not just to alleviate your guilt. He can't really allievate it either, you have to work through that yourself I think.

You know..I have been through something similiar...when it all came out in the wash later and I found out by chance... not only did I feel hurt and betrayed, I felt humiliated and like a fool...the idiot to laugh at while everyone does a cover up...just a pawn in their game. I also felt robbed of the choice to decide for myself. Was that fair? Someone telling me upfront would have been less devestating than choosing to continue the sham.

This is just my opinion. But I feel it is wrong to betray a friend further. That would make that whole saying who needs enemies when you have friends like that pretty true. I don't know there will be an easy way to tell him, it's pretty heavy. But I think your are doing the right thing.