What her reading, tech and budget preferences are will determine how "open" she can be.
Would she want to read mainstream authors published by the big 6 publishers? Is she willing to ADE and Calibre her books before she can read them? Does she travel a lot and want instant access to bookstores? Any budget restrictions? Willingness to purchase online and accept long distance service? Library ebooks?
Maybe she can test a few out locally and see if she likes holding and using them then work from there with the questions I asked and the other helpful posts.