Legal Question in Administrative Law in California

What does it say in the contract?

Probably not. You didn't state any reason, nor did you say whether this was a cash purchase or involved any financing. There are lemon laws and laws regulating financing, but in general a contract to buy a car is final and binding. Please note that both the dealer and the customer ordinarily sign a whole bunch of papers at closing, and your mom or both of you should go over these carefully to see what rights, if any, she might have at this point.