I also think by this time she realizes it is too late to help Greg, and seeing that she failed him when he was so young, she feels that respecting his wishes now at least makes sense.

It's good reasoning, but I don't know, I can't help feeling that a good Mum would go visit him, whether or not it was what he wanted. Jail must be an awfully lonely place. You'd think she'd want to see her son and make sure he was okay (and yes, I'm totally projecting my own awesome relationship with my Mum onto this situation :)