My mom cheated o my dad, what is the best approach to this situation?

My dad never suspected anything until one day he got worried about my mom and she would not answer her phone about a month ago. He was a little concerned and tracked her phone and noticed it was at her co-worker's house. His worry then turned to curiosity. There was no need for her to be over there especially at that time without telling my dad. My dad snooped in her as result and found some pretty detailed messages. They were mostly dirty and how much they loved each other. My dad was just sad after that. I have never seen him mope in my life and you could tell he was about to cry for a couple of weeks. Finally they talked and I guess they worked things out but my mom refuses to defriend this guy on facebook which is where most of the messages were coming from and she changed her facebook password so it is harder for my dad to snoop. My dad has a hunch they are still communicating but isn't exactly sure. I know my mom is crazy so I can't exactly ask her about it. I saw her grab a knife when my dad confronted her about it the first time. She has had depression on my life and she is scary sometimes. I think my dad should just lose her. I mean my mom just seems heartless anymore. What would you do?