If it had been me, I'd have leapt straight for the following question: "What do you need it for?"

I would add, as a clerk in such a store, her asking you for help is not her asking you to do her job, it is asking you to do yours. Not everyone has knowledge of such things. She may very well have an in depth working knowledge of Medicine, for instance, but be completely ignorant to what the difference is between "chiffon" and a "poly-cotton blend", and would need the guidance of someone who can tell her, "Oh, you don't want that one for a kindergarten school play, it'll tear too easily"

Who knows? The thing is, a well placed question would have answered this and more, and is that not what those of us in the service industry are all about?