My moms cheated on my dad? :O?

Okay my mom and dad have been married for over 30 years now. My mom and me have been getting close over the past few weeks talking about things like Making love, funny things, gossiping, shareing problems and she happened to disclose this secret where she had cheated on my dad when he was at work with his business partners brother. This was years ago almost 20 + years ago and she has never told my dad. And I love my dad but I don't know weather I should let him know about this cause its been 20 years to that thing and shes pretty guilty about it. I don't really want to spoilt there relation but its kind of sickening. What should I do??!??!