If she admitted to the police that she was guilty then that will be on the police report. As said, she ran into the back of a stationary car, it is going to be almost impossible for her to get out of blame.
There is nothing you can do it is out of your hands , just let the insurance sort it out.
Just to repeat it is NOT the police who decide to prosecute or not , it is the CPS (Crown Prosecution Service) , the police just submit the report and the CPS decide if they are going to charge the person or not. The police have nothing to do with it apart from putting in the report.