People love to hate the 9mm, but it’s made an awful lot of bad men permanently good. You do want a good HP though. 9mm ball can zip right through up close. Bottom line, try out several calibers and see what you’re most comfortable and accurate with. The fabled “one shot stop” is not what you need to worry about; you plug the SOB until he drops or drops his weapon.
Hey dummy, tell him to get a Beretta 92. Should be plenty to choose from at a decent price. I actually can conceal one, and I’m around 6’2” with a lean frame.