Did they tell you how often they changed their water?
Most petstores don't keep up with water changes and if you're feeding a betta that's in a tiny cup everyday the ammonia levels in the water would be horrible and will definitely kill the fish.
As long as you keep the water clean, you can feed your fish everyday.
Some people go for 2 or 3 times a day, some with once a day and others don't feed one day a week.
Personally, I feed my fish once a day because of my schedule. Sometimes I have a bit more time in the mornings and they'll get one pellet or two. Then Ill feed in the afternoon again. They seem happy and I haven't had any problems so far. My fish eat everyday.