You will be presented with this dilemma through all your life. Freinds are people. They sometimes cheat, steal, have affairs and such. There is no one size fit all.
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The right thing to do would be to tell your friend so he can move on from his cheating GF and you. By the way, what you did doesn't make you much of a friend.
This question sparked one of my friends to mention how he's never had a blowjob because his girlfriend of two years isn't into that, to which Chole asks him do you.