I have a friend this happened to. He divorced his wife because she was banging his best friend from high school. A few years after the divorce she wanted to reconcile and they started seeing each other and he got very suspicious a few months into it. He worked second shift and never went home for his dinner break. One night he decided to and walking up to the house he peeked in through the window and could see action going on. He opened the door and the ex wife was being banged by the best friend on the living room floor. Just like old times. I rarely go this far in conversation, but, she is a c***. One of the worst women I've ever met. Oddly enough, he didn't ditch the friend. Very weird dynamic.
The very worst thing of it is she allowed for the kids to start getting used to the idea of their parents getting back together.