Unfortunately, pet store employees usually don't know what they're saying, and are only saying it to get more money. By putting in that many fish, you'll end up with deaths, and maybe you would want more, so you'd try again, resulting in the same. In the end, the pet store comes out with more money than if you'd simply put one betta in there, and everything had worked out, forgive the pun, swimmingly. Bettas are, as others have stated, solitary fish, and do much better on their own. If, however, you do want your little girl to have "friends", you could try a 10 gallon sorority. If you were to go that route, you must have a ten gallon tank, plenty of hiding spots and plants, and at LEAST 4 females (I've heard an odd number works best, though), and less than, say, 7 females. And no males whatsoever. Males and female bettas must not be housed together unless breeding.
Hope this helps a bit! Welcome to the forum, by the way!