MIL Cheating help!

So my hubby & I have been married 2 years together 3 and we have a child. My MIL sat me down one day and explained that she has been seeing multiple men throughout her marriage for the last 8 years. Immediately when I got home I told my husband and we have kept it from my FIL. We have both brought it to her attention that she needs to come clean. I have met one guy that's she sees let's call him W. well now she has gotten balls and brings W to the house in front of all her family and my FIL. For the 4th she had us drive to Ws house, pick him up and go see fireworks with our FIL. She tells my 2yo to call him uncle will which I am completely against. I get she's cheating but I don't want any part of it and I think it's disgusting my FIL is the sweetest man. My question now is its putting so much stress on my husband and I because we want to tell my FIL but we don't want everything to erupt. What do we do.? 😊