Discharge By Objective Impossibility

Penelope saved her money to undergo cosmetic surgery. She signed a contract with Dr. Shapiro, paid him in full, and scheduled the operation for the following week. Prior to the surgery, Dr. Shapiro suffered an unforeseen accident, severely burning his hands. As a result, he is permanently unable to perform surgery.

On what legal grounds can Dr. Shapiro be discharged from his contract with Penelope?