The Kindle is the simplest, because she can buy books directly on the device (if Sprint is available where she lives). It is heavier, and the scroll wheel and keys might be too small. The page turn keys are if anything too easy to press.
Assuming the page turn keys are ok, the only downside to the Cybook is that she will need to use her Mac to buy and download the books. Once the books are downloaded to the Mac, they are transfered to the Cybook as if it was a USB memory stick.
Note that you can buy Kindle books from Amazon on the Mac, and then either download them to the Mac and then to the Kindle (similarly to the Cybook), or just turn on the Kindle's Sprint connection and they will download automatically.