You have to stay out of it.
1st, you don't know that your dad was doing something that your mom was not aware of. There are many marriages where a husband or wife seeks lovers with PERMISSION. In that case, it's clearly not your business.
2nd, it could be that dad is trying to be sneaky. If so, then he's stupid as mom will find out eventually. And she won't need help from her child to do that.
In a worst-case scenario, this is an issue for your mom to work out with your dad, if you can. Your input will not be welcome by either your mom or your dad.
A marriage is just between two people. You don't have the right to interfere in the marriage just because one of them gave birth to you. If you say anything, it justs adds another level of complication to a situation that is already pretty messed up.
Your mom does not need your help. Either she knows already, or she will soon.