I missed the part where he was shooting straight up.
If it's high enough that no one is going to run into it, he should have left it alone. It's not presenting any threat to him.
Self-defense is a right. Shooting at things that merely upset you is not.
Provide the angle of the gun to the ground is sufficient that nothing will be in the path of shot until it reaches its maximum height, than nothing can possible be struck at greater than terminal velocity.
Not strictly true. Two exceptions.
A shot fired parallel to the ground will have maximum velocity at its maximum height, and a shot fired at very slight elevation can have its vertical velocity cancelled while still remaining above terminal velocity. Granted, it would continue to slow while falling until it reached terminal velocity.
But for a guy shooting almost straight up---yeah.