The way you are describing is a bit difficult, but not impossible. If I remember correctly this depends on whether or not you have an old partition system on the NC (can see all the folders, even the ones specific to B&N) or the new partition system (can only see your content, not the downloads from B&N). If you have the old partitioning you COULD navigate to the B&N area, find the books folder, and search out the correct book via ISBN number (a pain).
I think that the easiest thing to do would be to have your wife download the book to your computer from her B&N online library (B&N, my account, my NOOK Library, select the blue download button next to the book of your choice) and sideload that onto your NOOK and enjoy. (Note:if the book is DRM protected it will prompt you to enter information when it opens, but you only need to do that once for the book)
I hope this helps.