Or.... correct the problem.
are you going to adjust sights on every gun he buys from then on ? Thats one of the reasons its important that the instructors know how to shoot. All it would take is one or two rounds fired by someone who can shoot to either confirm or disprove if its the gun.
If everyone fires the gun and it doing tight shot groups in the same spot then adjust the sights (if you can or if you can without screwing up the gun)
But if its not the Gun (usually the case) then the shooter needs to be corrected (thats what any good instructors are supposed to be able to do after all)- never adjust a gun to solve a problem caused by the shooter
in this case I think a little ball and dummy drill would be helpful.
never adjust an accurate gun - never offset aiming points (ky windage)