Sorry to hear you're going through this. Divorce is a very difficult thing, not only for the husband and wife, but for everyone else nearby.
Hopefully your mom won't make things even more stressful by screwing your dad out of half his retirement and life savings, and then demand alimony as well. Men often come out on the short side of the stick in these matters even when they are not the one initiating things. If she seems the type, he needs to start talking to a GOOD lawyer immediately, preferably without her finding out prematurely.