Good one dunerunner.
What the gun owner was trying to show is true for most guns, such as the 1911 and Glocks. When the locking lugs on the barrel and slide disengage the pistol will not fire even if the trigger is pulled. Some people pratice dry-fire and aiming technique with a Glock by folding a piece of paper and putting it between the battery and the top of the slide so it disengages the trigger and moves back and forth because of the trigger-return spring, but without resistance from the sear. Of course when you do this, you always make sure the firearm is unloaded twice over.
Yet another negligent discarge, not an accidental discharge.