Personally, I dislike B&N's DRM scheme (credit card number). You might be able to get a gift card from Amazon or Kobo or some other store.
If she is reading on an iPad, there is no reason to restrict yourself to ePub. Amazon has a good selection.
However, if Apple's new demands make the bookstores remove their apps altogether, you might have a problem. Certainly iBooks will remain, but their selection is limited. If you are computer-savvy enough to strip the DRM from Amazon or B&N books, etc., you could at least back up her books for future use with an app that she could still use on the iPad.
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