It's his fault for not training her adequately. Just as you can't assume someone will understand the risks of getting behind the wheel of a car for the first time without training them, gun safety is not implicitly known by first time shooters. There are a number of things that need to be said in addition to just the four rules. Many come to mind.

• Take your finger off the trigger as soon as you aren't looking down your sights (everyone assumes they have good trigger discipline if they don't know exactly what it means)
• Never try to catch a falling gun
• Never turn around with a gun in your hands
• Never put your thumbs in the way of a reciprocating slide
• Stop firing immediately if you expected a bang and got a pop or it didn't fire. Hold the weapon on target for 30 seconds in case of hangfire.
• Stop firing immediately if you got a much bigger bang than expected, your ammo may be dangerously over pressure
• If you get hot brass down your shirt, keep the gun pointed down range at all costs

There are also a ton of things that can cause (potentially dangerous) issues when handling and maintaining guns too

• Putting a bullet up the feed ramp multiple times can set the bullet back into the case, causing dangerous over pressure
• Getting too much lube in the bore can cause over pressure
• Getting lube on the striker in a handgun can cause light strikes, leading to hangfires
• Touching your face during or after shooting can expose you to lead. Special soap is required to fully remove the lead.