Hi there,
Can relate to that, I know how you feel. All in all, I can just say, be proud of your mom and support her. There's no need to think anything bad of her, or about her man. Whatever his intentions might be, I suggest you respect and welcome him.
My mum went through similar thing, though she left my dad long ago, but had few boyfriends later, but they mostly exploited and used her. Then, like 2 years ago, I started working for a small bussinessman occasionaly, and he met her through me on an occasion. He immediately liked her a lot, asked me about her, and soon he wanted to take her out. She didn't wanted to go out with him at first, said she isn't looking for anyone, but I've encouraged her to atleast give him a chance to take her for a drink or dinner. She did, and long story short, he is dating her and they're practically together now since then.
Though, some people were jealous, locals that knew her, even commented that ''she sold her cunt pretty good'' and such things.. He's 54, has money, she is a good looking 43 yo woman, you know the drill... But I didn't mind, I know she doesn't spend his money, though he does take her on vacations more often and things like that, which is good.
I'm proud he has her and I respect him, we're in very good relations.