May I suggest a .40 S&W COR-BON 135 gr with a 96%+ "One Shot Stopping Power". That should do the trick so he doesn't have to waste all of those rounds.
A fine choice. I favor a Charter Arms Bulldog loaded with Cor-Bon 165 gr. JHP's. To each his own.
That's what I load up my S&W "Shorty .40" with. And if not, I just go for head shots.
Roughly the same as either a Federal 9mm 135 or a Remington 147 9mm, both of which probably have a little more penetration than the .40. The forty is fine if you like it; I prefer a 9.
If I was deliverying still, I'd carry the Kel-Tec .32 I carry now. Wasting rounds, as you say, is often more a sign of bad shooting rather than underperformance of the chosen round. And we don't know which handgun, though I'm guessing by the 15 rounds fired, it was a Glock. Cheaper hi-caps available, and Sigs are probably too heavy for pizza carry.
Better yet, he should get some more range time, shot placement trumps caliber.