In divorce does 50/50 always apply?

[Taso]

This is personal, but no one on this forum knows me really, so I figure screw it.

Basically, my mom cheated on my dad around 2 years ago, and she still is, with the same guy. My dad is a doctor, works three jobs, and owns four houses here in America, and a few in Greece. He's worked (and still does) very, very hard, even though he is sick, to make money and to keep the properties going (As investments). It would be sick and twisted if my mom, who is a manipulative, terrible person, were to get 50% of his net worth as the result of a divorce - because of this he has not divorced her, and has to live with her [censored] every day. He won't talk to a lawyer, I don't know why, he's stubborn. It's a very tough situation, but both my brother and I (both over 18) would not have a problem making a statement to the effect of "she had an affair and is evil".

So I'm asking any legal people, or anyone who knows, would she be entitled to half of his assets in this situation? Does the affair break any contract they made by getting married? What's the deal?

thanks