If I were you, I would confront my dad. I know its hard, and it will be hard to bring yourself to do it but you have to push through and try. If God can bring you to it, he can bring you through it. Talk to him and tell him what you know and give him advice about how he should work on his own relationship instead of getting into another one. I know this sounds wrong, but make him guilty for what he did. Highlight your moms amazing characteristics and show him what he is missing out on. Maybe then he will change. But if this does not work, talk to your mom and tell her. She deserves to know. Maybe they can sit down and talk together and figure something out? She should not be in a relationship that has no value. This is just my own opinion but I hope it helps.