I might get downvoted for this but here I go:

First of all, I am not trying to rationalize your dads actions, but I do want you to understand the male brain. What he did is wrong but please read the following:

Men are different than women when they cheat. When a man cheats, he is just looking to release some sexual desire… that’s what we’re wired for after all: to spread our seed. When a woman cheats though, it’s usually because she built an emotional connection with that man that ain’t her main, so she no longer loves the other guy. You can say “naw it’s the same thing” but I disagree. Men can have sex with someone and not get emotionally attached (this is why men are better known to sleep around more and do one night stands than women.. This is why men biologically have that “post nut” mental clarity because they want to “get away” after impregnating the woman and avoid responsibility.. onto the next girl)… this is primal instincts which are not acceptable in society at all… AGAIN, I am not trying to convince you that your dad did nothing wrong, but I am trying to help you make sense of what was perhaps going on in his head when he did such thing.

When a man cheats and falls inlove, he will usually seek to get a divorce.. but according to your situation, he never did that with your mom… but I cannot make any judgements because Idk their story or yours.

As for if you should tell: I think you should start with your father first, not your mother. Try and see what he’s doing now. Say something like “I found out my friend’s dad cheated on her mom, pretty messed up isn’t it dad?”… and then build up to it and confront him.. ask him about it and why he did it (most likely for the reasons I explained above)… I don’t think you should start with your mom before doing so with your dad and analyzing him.