Laws vary between states.

1st thing) I would verify who actually owns the house. That is not clear from your writing. Whose name is actually on the deed title? Because as far as the law goes, whoever is on the deed is the owner.
If it's under his name only , even if he bought with her help - then it gets hard.

2nd ) if the house is not under your mother but she helped him substantially in purchasing the house, you may consider filing a civil lawsuit. Better get a lawyer with this. It's complicated to do it on your own.