CPU problem?

My Son in Law bought a Fiat car from Carcraft about 18 months ago and the immobiliser went wrong.
The AA couldn't sort out the problem and so a local small company was called out to find the problem but the car was towed away for further diagnostics,The resulting diagnosis was that the entire CPU was no good and to replace it would cost anything up to £2000 plus labour!
As he has only done 17000 miles from buying it,surely the onus would be on the company who sold the vehicle should attempt to find out what part /parts can be repaired.
The AA,who they are insured with will pay £500 toward repairs but they insist on the Fiat Dealer doing the jobbut this will cost £300+.Could anyone advise on where to go from here as the car is still being paid for.
I belive the car is just 3 years old and wondered about the lifespan of the CPU Unit..any advice please?