The Rosenbergs sold their Dairy Queen franchise to Mary Pratt. The contract required Pratt to pay most of the purchase price over a 15-year period. Two years later, Pratt assigned her rights and delegated her duties under the sales contract to Son, Inc. The Rosenberg's agreed to the new arrangement. But the agreement made no mention of discharging Pratt from any further liability on the contract. When Son failed to pay, the Rosenbergs sued Mary Pratt. What result?