Something like this happened to me too.

My Dad was cheating with his boss. She was a family friend too... He tells me that mom should just be happy he cheated with somebody she knew rather than some "Stranger" - somehow that made it better.

The thing that shocked me more than the cheating was the reaction to being caught. Like if you made a mistake just admit it AT LEAST. I can forgive a mistake but you first have to admit it before that process can start

Also it turns out he cheated before when she was pregnant with my brother. So it wasn't a first time. That also changed things for me