I have been there. There is no magic solution.
I heard similar when I was a child. I have now been married 15 years and realise that no marriage is perfect and we all have secrets that would shock people outside our marriage.
A few years ago mum did open up to me. She didn't like my answer. Mainly because I knew they had both cheated.
She wasn't happy that I knew that dad wasn't the only one who had fucked up. Also both of their mistakes happened over 20 years ago. She was still punishing him, for his mistake. Though he had let hers go.
My opinion on infidelity is that if someone cheats, it's awful. But you can't keep punishing the cheater forever more. If she couldn't move past it (or even understand it having done it herself) after 20 years she needed to leave dad. Spending the next twenty years miserable about it is no life for her.
Also being punished and verbally abused for a mistake that happened twenty years ago, is no way for dad to live either.
I think mum expected me to join her punishment of dad.
I love her but our relationship has suffered. But something sunk in as their relationship has been getting better.
Sorry for the long post, but what I am trying to say is a lot revolves around how your parents relationship with each other is, and how your relationship is with them.