In order to fight an attorney, you probably should have an attorney of your own. If you are in the U.S., the BAR association will refer you to three attorneys in your area that specialize (in this case, I would guess it would be contractual law) and they are required to charge only a nominal fee for the first meeting. A verbal contract can be binding, but there is also the consideration of the attorney being able to unduly influence your mother's decision to enter into the contract because he is considered an expert and your mother less knowledgeable. You wouldn't have to prove she was mentally incapable of making that decision, simply that he used his position to coerce her to sign a contract that wasn't in her best interest.

Of course, that only comes from a business law course in college and having a brother who's a lawyer, so definitely get the opinion of a real attorney! Good luck.