betty is immature, superficial and a bad mother. it's hard to see how don ever loved her beyond her trophy wife exterior. the thing that made this episode so good -- like the other california episodes -- is that he gets to be dick, without work, without having to even come home and not be relaxed, and he has someone who loves him unconditionally.
so to hear she was dying of fucking bone cancer... she has to know, innit? she would know SOMETHING was wrong if it was terminal. i think she was playing along.
and i think don hasn't fucked joan because they're close in age and she has a way about her that wouldn't allow that to happen with someone like don, an entirely different animal from roger. she knows him too well, and we saw how old-fashioned don's romancing is. the niece wasn't having it, AT ALL. and that other blonde from the first episode was playing a new game, also.
i feel like this is the beginning of the end of don. at a certain point in season 3, he might have reached his peak.