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.