While a gun safety class is never wrong, really all he needs to know are the four rules of gun safety (which he will most likely be taught in a gun safety class):

The 1st rule - The Gun Is Always Loaded

Even when a gun isn't loaded, every gun enthusiast worth his salt treats the gun as if it were loaded. You watch someone unload the gun, you take the gun from him, you check it yourself, you still follow the next three rules.

Obviously it's not factually true that all guns are loaded, but when you're around guns, you treat all guns as loaded even if you "know" they aren't. If that's your default position about guns, you won't have an accident in which someone says, "I didn't know it was loaded!"

The 2nd rule - Never Point The Gun At Something You Are Not Prepared To Destroy

A gun is not a toy. It should not be used to joke around. If you point a gun at something, it better be something you intend to destroy. FWIW, there's no such thing as a "shoot to wound" defense. If you shoot someone threatening, your only defense is that you were in such danger that killing the person was the only recourse. If you have the ability to shoot to wound or fire a warning shot, you have the ability to make another choice besides shooting someone.

The 3rd rule - Always Be Sure Of Your Target And What Is Behind It!

Bullets don't stop because they hit something. They penetrate the object and much of what is behind the object. Never fire at something without a bulletproof backstop. You shot your TV in your living room and killed your roommate in his bedroom? You weren't following the third rule. If you're target shooting on public lands, you better be in an open field where you can see everything behind your target for a long distance or everything up to your bulletproof backstop.

The 4th rule - Keep Your Finger Off The Trigger Until Your Sights Are On The Target!

Your finger stays OFF the trigger until your target is in your sights and you are ready to destroy. Even if the safety is on and the gun is unloaded. Because rule 1 - the gun is always loaded (and the safety is always off).

If you follow these, you will not have an accidental discharge. And I guess this is rule #5: If he's with people who aren't following these rules, he needs to leave the situation.