IF i was you, i would tell him. I know it will not be easy to do, i am sure that he won't believe you and it will probably turn pete and his girl against you. How do you know that she is having an affair? do you have enough proof to be able to convince Pete? do you know the name of the guy who she is having an affair with?
Personally i think that in life you regret the things most are the things that you don't do, rather than the things that you do. If you tell Pete, then at least you will get it off your chest. Ok, so he may not react well, but at least you will know that you have done what you can for him.
MB