I would confront the one who is married. tell them they have one week to stop or you will spill the beans.
i know there are many who say stay out of it but how would they feel if there partner was having an affair and the person could have stopped it or at least warned you about it but didnt?
it is unfair and, if they are in the house with chidlren completly iresponsible to put children in a postion where theyt o may discover and have to lie!
they are both disgusting people whom i woudl no longer consider freinds.