Actually, since the DRM can be handled by Adobe Digital Editions and there are various ebook stores around that support Adobe ePub/PDF, he can use completely different log-ins for the purchases and activation.
If you want to read a book on a reader authorized to adobe id "toto", you have to purchase and download the book with a version of ADE authorized to user "toto".
A book downloaded one with a user cannot be downloaded with an other adobe id afterwards.
And there is a 3 time download limit.
Spare yourself some pain, liberate your books.