this should be easy. Deauthorize on her account authorize on your account. add the books (to an SD card) and reverse the whole process after you REMOVE the SD card.
now if you stick that SD card in her book (maybe either book ??) it should work just fine UNLESS some kind of key is stored on the reader itself they when you reverse the auth those keys may chance and the readable books may not read any longer. the only way to know is to try. its no harm to try so give it a whirl and let us know what happens.
the right solution is to not buy DRM'd books. then its not an issue.
Chris Taylor
http://www.nerys.com/