I realize this did not occur in Texas, but under Texas law I believe the gun owner is culpable under criminal negligence provision. This is certainly NOT an accident.
Texas Penal Code Sec. 6.03. DEFINITIONS OF CULPABLE MENTAL STATES.
(d) A person acts with criminal negligence, or is criminally negligent, with respect to circumstances surrounding his conduct or the result of his conduct when he ought to be aware of a substantial and unjustifiable risk that the circumstances exist or the result will occur. The risk must be of such a nature and degree that the failure to perceive it constitutes a gross deviation from the standard of care that an ordinary person would exercise under all the circumstances as viewed from the actor's standpoint.
Far as I'm concerned they broke ALL FOUR rules.
1. All guns are always loaded.
2. Never let the muzzle cover anything you are not willing to destroy.
3. Keep your finger off the trigger until your sights are on the target.
4. Be sure of your target and what is beyond it.