What do I do? Should I tell him I know?

Wow Jen sounds like a ho, but anyways i think you should tried to channel you thoughts from what you see. For instance you're experiencing this all at a young age try to become an activist against it and advocate a life of non-cheaters or helping woman cope with men who do things of this nature. I think you should confront you father, not about the emails as that would be invading his privacy but about the blatantly obvious with Jen. Be respectful and explain to him how his wrong-doings have affected your life and how you perceive him. Anyways hope i could help, it sounds like quite a situation, keep your head up and persevere. :-)