My dad is having an affair, what should I do?

DO NOT GET INVOLVED! and make it very clear to your parents that you don't wana hear about it from either of them. I was in your exact situation a few years ago and my parents dragged me right into it because i wanted to help and I wish I would have just ignored them the entire time. That's their problems not yours and they should know better than to gossip to you about each other cuz you're their child not a friend or coworker. Don't worry or think about any of it you can deal with it later if/when anything changes.