Wolfie, I know this sounds childish, but sometimes childish is best. There MUST be some way to pay the b*tch back; report her for some sort of discrimination. Your mother is not insane: just mentally unwell. Therefore, her employer is discriminating in firing her and is guilty of lack of tolerance for her condition.
Otherwise, you may need to take it off the common way. Put fish in the wench's curtain rods.
If you don't want to do either of these, the final option: Wait & Hate. One day a chance will more than likely come up and you'll have your own back then. Or maybe something will just happen to her and you'll be there to mock her loss.
Whatever way, tell your mother she's the better person and it's not her fault.