To follow what glocker said, there is a reason the 9000S was discontinued. It was problematic from the day it hit the market, and the first batch had to be recalled due to a problem with the safety lever. To the best of my knowledge, it's the only pistol ever rated as "don't buy one" in a Mas Ayoob article. Seriously, they were spectacularly craptastic.
I've described the 9000S as "the kind of gun you'd expect from a country that hasn't won a war since Rome fell."
The problems your girlfriend is having with the 9000S are almost certainly the gun's fault and not hers. Make sure the pistol is completely lubricated per the owner's manual. You could try shipping the gun back to BUSA, or you could just find a better gun that fits her. Weaker ammunition is not likely going to fix the problem, and if anything I expect might exacerbate it.
To answer your original question:
The softest-shooting factory 9x19mm ammo I've shot is Cor-Bon Performance Match, which is a 147gr bullet at 850fps.
My current regular practice load is 124gr "Indoor Range" lead-free TMJ from American Eagle, which is also unbelievably soft shooting, cleaner than any ammo I've ever seen, and very accurate. It costs less than the Cor-Bon and comes from a major manufacturer.
Standard Winchester White Box is also notoriously soft shooting, to the point where it sometimes fails to function in tighter or more sensitive guns.
Train hard and stay safe!