Charlatan

In that case... I would stay out of it except to confront the cheating parent and say, "You need to do the right thing here, either break off the affair, or get a divorce."

I still don't think it's my obligation to inform the other parent about the affair. Their marriage is their business.

__________________
"My hands are on fire. Hands are on fire. Ain't got no more time for all you charlatans and liars."
- Old Man Luedecke