Minding your own business is my suggestion. HOWEVER, had a friend that I new his wife was cheating on him. My then wife would not let me say anything to him. He found out after we both divorced the jerks. He was very hurt. I regret not telling him, our friendship was never the same. I do not know what type of friend he is, if he really is a very good one..... Think about it. Its a personal decision. Sorry for the mixed message.