Sounds like the lash wasn't set correctly, & // or the guy took the RPMs way too high without being under load ( driving or dyno run ). RPMs climb fast without being under load.
** Of course I may be way off on my opinion.**
Personally I don't rev above 25-2800 in park or neutral. That's plenty for checking for vacuum leaks, throttle response, checking for fluid leaks, fuel pressure, fuel flow from a carb, listening for odd noises.
Spring pressure, valve float, piston contact come to mind.
Any kind of knock or tap prior to the event?