My friend is cheating on his partner.What would you do if you were in my situation?

although it really isn't your place to say anything, i would almost definitely tell my friend who is cheating to knock it off or you will tell his husband. and if it does come to that, try to be real gentle and you might want to bring along that reliable source to back yourself up.or you can just let it go and eventually the husband will probably find out on his own the hard way. good luck with your choice!