Here are a few that you might try. None of them seem to do Kindle books in particular, so she would have to do the DRM removal and Calibre conversion (as per Veezh, above).
http://www.bol.com/nl/boeken/nederla...ken/index.html
http://www.ebook.nl/store/
http://www.libris.nl/?boeken
http://www.eboekenweb.nl/nederlandse-ebooks/
I've not direct experience with any of these.
Mike