I caught my dad on Ashley Madison. What should I do?

He I married for 20 years and has 2 kids. We were eating sunner and I had to go into the den to get something and I saw the website up on the computer screen. I know that he bas an account on it because it said his account number on the top of the screen. This also hasnt been the first time that I caught him. I havent said anything to my mom or him yet bet I'm wondering if I should do something or just leave it be.