Since the boys are in good hands, and you don't want to tell him, then I wouldn't!
Remember this summer when I went away for three weeks to visit my daughter? I was so concerned about her coming into my house, and being around my boys. In my case, my boys are my dogs, but just the same I didn't want her touching them. I still don't know if she did or not. There's nothing I can do about it now anyway. I just remember that GOD is in control of the situation, and put my TRUST in HIM.
My advice also would be to leave the FT alone. She will do herself in eventually. She has no morals. How can she, she is with a married man. No matter what he tells her, and she chooses to believe, she STILL is with a MARRIED man. Let GOD be your vindicator.Let HIM have the GLORY. Keep your INTEGRITY, and you'll be the BETTER person for it!
Believe me I know, it's not easy. Oh what I would love to do to the skank! But she has no morals. She is a sick person. AND She is not in GODS favor.