Leave your dad alone.
Tell your mom to accept it. Your dad has done most of his responsibilities since all of you are grown up. Take care of your mom and shower her with love and affection. Get your dad to set his priorities right as to the distribution of his assets, if he has any. Get him to transfer the matrimonial home to your mom.

Blame the father for all you want. I also don't condone this kind affair. A fact is a fact and what is done cannot be undone. If TS thinks that the family will be better off without his dad, by all means take the necessary action. It's not just your mom decision, it's a family decision.
Well, please note that most men reaching the age of 40+ are likely to stray and it's advisable for their spouse to keep an eye on their movements and behavior. And men over 50+ are more attracted to china dolls but fathering a child is really stupid and dumb!

One day, while driving in town, I saw my dad's car coming in the opposite direction. Gosh, I noted there was a woman sitting in front and I said to myself, what the heck my dad was doing. At that time my dad was already retired and we are all grown up.

When I reached my office, I have no hesitation and called my mom. I told her what I saw and I told her its alright what dad is doing and if there are women stupid enough to "stick' to my dad, its her fxxing problem, not ours. I said dad is already retired and maybe its better for him to keep his reflexes active and his mind young rather than sit at home and wait for his calling.

The only thing me and my siblings told my mom is that make sure he's is not stupid enough to let that woman "con" him of all his savings.

The good thing is that he knows how to come back home and he has not produce another younger issue.

Sometimes, its not bad; just look at the positive side of things.