Engine issue after being rear ended

I am in need of assistance of an explanation and I hope that someone can help. I have a friend who was rear ended while sitting at a stop light. I have driven the car prior to the accident and the car drove smoothly and it was so quiet you could not tell it was running. My friend does not drive her car often and when she does it is around town.
After the accident she had to go to an appointment that required her to have to take the highway, while driving on the highway, the car began to knock and lose power. She had to pull over and we had to have the cat towed because the car was not drivable.
Prior to the accident, there was never an issue with this type of problem.
The conclusion that we came up with is this happened from being hit, Could you please provide any assistance, as it is greatly appreciated,
Thank you