First, there's the e-ink vs. LCD. Before you go too far studying things, I would see if she/you like reading books on an LCD because if you really prefer e-ink, then that narrows the discussion considerably. For e-ink vs. LCD, you can look at various readers at Barnes and Noble, or at Best Buy and try reading a page or two. Personally I don't care which one I use but I know a whole lot of people who only like e-ink (and a lot of them are here on MD&G). If you (she) really only like e-ink, then get a Kindle Touch, or a Nook SimpleTouch.
If LCD works well enough for reading, then there's the question of "do you want an e-reader, or do you want a tablet". For me, as person who travels a fair bit and likes to watch movies, and play games and surf the web, as well as read, I'm into tablets, but if you don't see yourself doing a lot of that stuff, then maybe you just want an e-reader. But if you do decide you want a tablet, then the next question is whether you want 10" or 7" - some people want lots of screen real-estate (me), but others like the lightweight 7" tablets because the larger ones get heavy when you hold them for a while. And then there's the question which eco-system you want to tie yourself to, Apple or Amazon (or a mix - Amazon has much better books, but you can get Kindle for the iPad and solve that problem). And lastly - but depending on your budget maybe it's even more important than the rest of this, there's there's whether you want to spend $400 on an iPad 2 (or $500 for a 3) or $200 for a Kindle Fire. Or $100 for one of the e-ink tablets.