You are doing what your father is doing. You are asking, "What should I do?" as if that is the question. He asked the same question, and look where it has him.
The right question is, "What would you have me do, Lord?"
And, just so you know, I do know what you're going through. The two differences between your story and my story are that Mom left him for several reasons, and that was one of the them. And it is several decades later, so I know how my story worked out.
And my biggest regret? I didn't ask God "What would you have me do, Lord?" for eight years, and didn't catch on to part of his answer for a few more decades. Because of that, family secrets are so deep none can come to the surface because they affect the innocent. The guilty have managed to harden their hearts enough to not even remember there are secrets.
Your father cheated on your mother. I do know how bad that is, especially having to live with the cheater as family. BUT is that better or worse than harboring such hate? Which is the worst sin -- breaking the fifth or seventh commandment? According to Jesus, there is no worse or better. Breaking one has the death penalty.
Following him takes that away.
So, ask the right question and seek the Lord for your answer.
And, if interested, I can tell you the thing you are falling for that caused me the most frustration for the rest of This thread was simply to get other Christians perspective. I'm not harboring hate or extreme resentment against my father. I'm allowed to have concern over his future as well as how it effects mine and my kids. I pray over this daily. I'm not faultless but you are seemingly turning this into my personal flaw.
Thank you for your perspective though. I do appreciate it.