What should my mom do about this?

My mom had a car a few years ago that she couldn't make the payments on, so she asked for it to be repossessed. They took the car back and already sold it to someone else but a few months ago they started taking payments out of her check for this car that she doesn't even have anymore. If she would have known this would happen she would have just kept the car. A large chunk of every paycheck is being taken. Is this even legal?