hiya, i havent been in that situation but from an outsiders view.... it sounds very much like he is still with his wife??? sorry to say that but are you 100% sure they are really split up, i know its been 2 years, but surely the connection between them should only be through there son? seems like his ex has some sort of hold over him which is why he is constantly letting your mum down, when he doesnt turn up does he give a valid reason?
i feel sorry for your mum, but 52 is no age at all! she has plenty of time to meet someone new, but like you said she loves him and i doubt anything you can say will change that unfortunalty,
your right to be worried but i think the most you can do is be there for her,
do you know anyone else that knows him? maybe find out a bit about him, see what others think
good luck x