If you really want to turn this guy inside out, and just to mess with him, you may want to confide in him, man to man, that you owe it to him to let him know that a few weeks (months ago), you had run into the nanny at a local watering hole and well, ya know, after a few drinks you ended up in flagrante delicto with her.
Watch his beady eyes sparkle as he grasps the timeline that could deliver him from this personal hell. As an upstanding guy, you just wanted him to know that it was a one-and-done thing and to not think too harshly of you as a person and freind.
Now sit back and watch the fireworks, He will tell his wife Pooj boinked the nanny, He will accuse nanny of being a two timing trollop, he will think he is in the clear and begin to attempt to recant his admissions and cross-cover his stories to fabricate new ones. Once he has swallowed the story and built a new reality for himself, (give him a week) call him and say "Just messing with you Tim, you are still a putz". As a parting gift you may want to give him Maury Povichs phone and email address too.