How to prove the other party is 100% at fault

I was in a parking lot, I had backed all the way out of my parking spot and a car ran into the side of my car spinning me sideways. My insurance found him 100% at fault bc the damage is to the side of my vehicle meaning he ran into me. But his insurance company says they will only accept 50/50. They said if he had hit my passenger door they would accept 100% fault, which he would have had he been in his lane. The only reason he hit the rear side is bc he was in the on coming traffics lane.