Poor little fellow! And poor mum! But I doubt the boy has any idea that his father "rejected" him, he's only 2. If he even has that concept, I'm sorry, but chances are his (angry?) mum put it in his head. She needs to think of her child first, and keep her bitter feelings in check. She's not hurting anyone but her child if she lets him think he was "rejected" by his dad.
If he's asking where his dad is, I agree w/ others that it's best just to tell him that his dad can't be there now or something like that. IMO, she shouldn't glorify the man, but she shouldn't badmouth him either, otherwise, as the child gets older, he may come to fear that he may have this bad side in him too.
I would say as little as possible for now. Time enough to talk/explain more ad the child gets older.