The novel makes no mention of a mistress, and the only reference to a mistress, in GFII, doesn't identify her. But, Sandra wins by default: I can't imagine Tom spending enough time away from the compound to even meet another woman, much less woo her.

At one time I was going to start a thread: "Did Tom have a life?" I didn't, because he didn't. Can you imagine Tom telling Michael, "Hey Mikie, I'm taking Teresa and the kids to Disneyland. See you in two weeks."