357 Sig is going to perform better than 9mm for any given barrel length. Given that, the point of 357 Sig is to go faster to hit harder, which, as noted above, is mitigated by the shorter barrel. If your friend wants a subcompact Glock that's more powerful than 9mm, I'd recommend they consider the 27 in .40 S&W, which doesn't depend so much on velocity for its power.
For what it's worth, I have a G32 and love the way it shoots. I also had a G23, and compared .40 and 357 Sig directly and found their recoil to be pretty similar. Some folks are more sensitive and have a preference for one over the other, but I don't.