re: Your girlfriend's best friend is being cheated on by your best friend.

THIS is definitely not your job to police imho. Your friendship is what you don't want to lose. It is truly those two's responsibility to make a good relationship. It is neither you or your gf's responsibility.

Reverse the shoe. What right to you have to information that wasn't entrusted to you if your girlfriend knew HIS girlfriend was cheating?

Your play is to let your friend know that you won't LIE for him... and how that is affecting you with your girl. That it's sorta making you feel guilty not telling your own g/f.

My thoughts are that if you have a stake to claim, then get involved, but it's not your bidness otherwise.

That's MY two cents!

Good luck justifying your actions on the opinions of others on a forum when she gets pissy with you for not coming forward!