gear shift was in park and allowed tth car to roll backward

My wife pulled the car out of the garage into the driveway. There is a downward incline on the driveway. She placed the car in park and set the emergency brake. She was getting out of the car to get something she forgot in the house. Half-way out of the jeep, it started to roll backward. It pinned her against the car and dragged her about eight feet backward and she fell in the driveway. This tore the skin off her knee and tore the muscles in her lower abdomen. She is in a lot of pain.