I think they are trying to argue that their sons car is not worth as much as it would have been if it had not been in an accident. I frequently have clients ask me about this. I don't normally handle this situation for them, but I do suggest they run comparables on kbb.com or cars.com to show how much the car would normally sell for pre-accident. Then, get an estimate on how much a car company like CarMax will give you to buy the vehicle.
If the offer is significantly less than what the value would normally be had it not been involved in an accident, and you can show documentation of that to the insurance company SOMETIMES they will pay you the difference. It all depends on the insurance company and the adjuster.