I opened a book to help a professor, now they can't open it.

I'm the technical resource admin for the College of Health Sciences at Eastern Kentucky University in Richmond ,KY. I perform many technical support functions. One of our Associate Dean's had just downloaded an ACSM book and had issues opening. They sent it to me to see if I could open it thinking it might have soem issues. My Adobe Digital editions opened it without question. So I visited the Dean and realized they had authorized their Adobe Digital Editions without an Adobe ID. I reveresed that, signe dthem up with an Adobe ID and re-registered Adobe Digital Editions3.0 with their Adobe ID. When I attempt to open "Their own book", it says it is already licensed/used by another user. Here is my question: How to I unlicense this book from my Adobe Digital Editions so they can open it and use it for their class instrcution? I do not want or need the book, I was merely providing technical support.