Did he ask with the intonation that he wanted them to fight? Or did he ask, possibly as a newbie just wanting to keep a couple of pretty fish together in a single bowl, 'having heard' they may sometimes fight, to which the Petco rep just offered a clueless response? The way you described the encounter, it's not very clear.
If the latter, I would've jumped in and said something to try and educate him. If the former, I would've followed him to the parking lot, got his license plate #, and filed a complaint w/ the ASPCA.
I dont know for sure but I felt like if he had to ask he must have heard about their rep before. I was more irritated that the petco guy didnt just tell him straight up they cant be together. Then when the lady at the register said to watch covering their air holes he just stack one on top the other like he didnt care.
Im pretty sure its because its how they can have a lot of them to sell with out taking up tons of room. If you think about it imagine 20 or so bettas that cant be housed with others all having tanks. Thats alot of space.