Auto Insurance criteria for backing accidents

What is the name of your state? Washington State.

My question -- Are there specific guidelines / criteria that auto insurance companies rely upon regarding accidents that occur when one individual is backing out of a parking stall and striking another vehicle, and more specifically when the damage is located on the rear quarter panels of both vehicles?

If the above question isn't specific enough, here are the details -- My son was involved in a parking lot accident in which a pizza delivery person, while backing out of a parking stall, backed into and damaged the rear quarter panel / tail light area of my son's car while he was driving past.

When dealing with the pizza delivery person's personal auto insurance regarding the accident, we were told that due to the statements received from our son as well as from the other driver, and also due to the described damage to the car, the insurance company was going to agree to pay to fix the damages done to my son's vehicle.

However, our insurance claim was denied because accidents occurring during the course of employment are not covered by the type of insurance he had, and this fact wasn't revealed to this insurance company until after they said they were going to pay. This is fine and well understood -- they aren't going to pay because he wasn't properly covered.

But for some reason the pizza delivery person wants to take our 17-year-old son to small claims court -- I suspect this is because he knows we are planning to take him as well as the pizza company owner to small claims court so that our son can get his car damages fixed, and that the pizza delivery driver's lawsuit is more malicious than justified.....

So....he is being sued, and as he is under 18 we are able to represent him. I have looked at all the Washington State traffic laws and can't find anything terribly specific about liability of people backing up their cars other than the obvious that one must exercise care.

What I really want is something official and in writing that we can present to the presiding judge over the case having to do with the automatic responsibility a driver has when they are backing out of a parking spot and hitting another vehicle, especially when the damage that occurs is to a rear quarter panel.