Or scope not holding a zero itself.  To weed this out, just pull the scope and zero the rifle with the iron sights instead to see if it still spraying rounds down range.


Also, might want to have him clean the bore with something like Sweets copper solvent to make sure that the bore is clean, as well as take a chamber brush to the chamber with CLP after the bore has been scrubbed to make sure that the chamber is clean as well.



Also, while he is cleaning the barrel, have him check the inside of the FS or Brake for bullet strikes to the inside of it.  Here, is easy to see, but since the rifle is used, might be a good idea to pull the FS off the barrel to clean the barrel crown, then reinstall the FS back on with a new crush washer.



About the only other thing that springs to mind, has a loose barrel nut that is allowing the barrel to wiggle around, or is putting too much pressure on the hand guards to torque the barrel during the shot strings.  To see if the barrel nut is loose, grab the upper receiver, and the Front sight base and see if you can twist the two against each other with any rotation movement.

And that you don't have a crapamatic scope in play that will not hold a zero.  I state this, since there a lot of scopes that will not even hold their zero on spring powered air rifle from there light recoil joint, much less a HP rifle (read are meant for cheap 22lr rifle that have no real recoil instead).