My wife got rear-ended, how can we prove she is not at fault?

My wife was rear ended leaving a parking lot and we have an exchange of information with a police officer. The girl that hit my wife at the time admitted to my wife that it was her fault but then told her insurance that my wife's car which is an automatic, sitting on a flat road, rolled back into her and damaged her car. She is now seeking to have my insurance fix her car and her insurance has stated that there isn't enough proof to place fault. They have pictures of both vehicles. How can I prove that my wife is not at fault?