Good arguments, and excellent replies!
She only has 3 ibooks, so they can easily be rebought, and given Apples Fairplay DRM can not be cracked (AFAIK), I won't be sending more money in their direction.
Good reasoning for Amazon, so a Paperwhite 2 may be a good idea (In part due to the built in light).. I like the 7 years of support of the Kindle..

Future proofing is good, but there's also a limit as to how far that actually goes. And she'd definitely have an easy time with the amazon store.
Thanks guys. The how-to-check-for-DRM-comments were also very helpfull!