As far as I know, beaks don't grow back. I know some hatcheries provide a de-beaking service, I actually had a couple of hens that had this, the top beak is shorter and rounded, making it impossible for the hen to do damage when pecking someone while still being able to eat and drink. Could this be what your chick had done?
No the store has given you the wrong information. They probably don't really know themselves, just guessing. Hopefully it isn't too bad, I have seen some real bad debeaking jobs. Some cant eat unless it's in a deep dish and some can't preen.
Keep an eye on her and make sure she gains weight. It is not her fault if she doesn't, they debeak so fast and so many that some get chopped off in bad places. You just have to make it a little easier if she has problems, but many do just fine.
I am not a fan of debeaking!