For those looking for a AA reader, the txtr beagle finally launched, it uses AAA batteries although it requires a Android phone (currently, could be expanded to Windows PC from what I understand, and I guess anything with bluetooth and extra application ability) for any effective use as it lacks ebook processing instead the phone processes the ebooks I guess in to images which it sends to the reader
http://txtr.com/beagle/
I'm not suggesting this for the OP or anyone else, actually considering the price which at 59 Euros is far above the promised 10, and not exactly that cheap even when you take off VAT it seems a bit silly to me. But since the topic of AA readers came up.
As for the OP, I would suggest the Mini primary for reason of price however you do have more options in the US, e.g. the ad supported Kindles for a fairly cheap price, refurbs. I do think some consideration of use would be helpful. For example I suspect something with touch would be important if she's going to be using the dictionary a lot. Similarly, would a larger font size be helpful? In which case it may be better prefer to go larger, so look at the 6" readers or even the Aura.