Auto Insurance Claims: parking lot accident, parking lot accident, certified mail

Question
My 16 year old son had exited his parking space at the local high school and was in the traffic lane, when 2 or 3 parking spaces to the right of where he was parked another student was backed out and struck my son's Ford Ranger in the passenger side door and caved in a 2 foot length of the bed of the truck while the other car continued to go in reverse. There was no damage to the other vehicle, only his vehicle's paint on the bumper.  The other drivers insurance company is offering to pay 50% of the damages to his vehicle, which they state is consistent their insured's liability from their investigation.  
I am confused, how did they arrive at 50% being my sons fault, when the damage location to his vehicle explains what happened?  Also, isn't the offer to settle to be made to the owner of the vehicle when it is only vehicle damage in the claim?

Answer
You would have to take the other party to court in order to get 100% of the damage costs. However, prior to that step you may want to send a demand letter to the other party by certified mail for all the damage repair costs, with a cc to their insurance company. See if that changes their position.

If you have witnesses to the accident, you may want to have them sign statements with your son's version of the events and send them to the other party's insurance company along with a demand letter for payment.