Poor thing. :( I had a similar experience in Petsmart today. The store manager told me I didn't need a heater for my bettas, and I corrected her right away. Cut her off and corrected her. I wish I worked at Petsmart, so I could give the right information. :(
I went by the betta section too, and the condition of the girls saddened me, and all the boys looked miserable. The dragonscales all had fin rot, one marble halfmoon had some kind of parasite, I believe, and all the others looked half dead or miserable. It makes me so sad. If I could, I would have picked up the marble. He was beautiful. I don't think I'd be able to save him though. :( Poor guy.
~ P o s e i d o n ~