He may have been dying already or they were super hungry. Not a normal scene but when you have small tanks, no cover and LOTS of fish, this stuff happens. And not just at Petsmart. Any store this could happen. Our Balas were well fed, I fed them twice daily when I worked my local Petsmart. Flake in the morning and frozen at night. Plus our Gouramis were all in other tanks. Our Balas were alone or only with fast fish.
You can always ask for the Pet Dept. Manager and ask that the Gouramis be separated from the Balas.
BUT sometimes customers pull stuff also. We had boys dumping Bettas in our Cichlid tanks !!! If I could have caught them and beaten them I would have.
Also rarely Fish sometimes jump into other tanks.
Stuff happens. Sorry the kid was traumatized.