The gun shop should charge him $2,000 for an education in gun safety with real experience included in what it feels like to be the victim of improper firearm handling. Education is expensive these days.
How much does one semester of college cost these days, and does a student gain as much value from their training as that former police officer did for the same amount?
Hate to say it but he may very well have a good case against the establishment for negligence.