My dad is cheating on my mom

Hey So, my dad has a job that requires a lot of business trips. Recently, he went to Boston for around a week and a half and my mom went with him. She got back on a Sunday night, I picked her up from the air port. My dad got home that Wednesday night. That Friday, I needed to log into my dads laptop to find a file or two I had saved from awhile ago. He has never told me to not go onto his laptop. He’s given me his password and stuff. He’s let me go on there before so I wasn’t doing anything wrong. I log in, and I go to the internet so I can upload the things I have saved on his laptop onto an email draft and send it to myself. In the recent pages I see a craigslist posting he’s been to lately and I click on it, thinking it’s a car he was looking at for my mom. It was anything but. It was an add he posted in casual meetings or whatever saying he wanted to get together either Monday night or Tuesday night for a jacking off buddy and a blow job or two. Said he was married and didn’t want any kissing or anal. The thing that made it even worse was that he was wanting it with another guy. (I know it was him who posted it because the page had buttons for edit post delete on there. If any of you know craigslist you know he posted that add) I showed it to my half sister (my dad was married before, cheated on his previous wife as well, and left her when she was 8months pregnant with my sister) She’s 25, and she said I should talk to him about it.

I can’t bring myself to talk to him. I know I could write him a letter telling him what I know. What should I do? Thanks much!