Apparently B&N may have changed some things in their system/apps for the worse, and you might have to switch the registration for your Nook PC app or change the info in your account just long enough for it to work.
There's a
post here from someone who seems to have had to deal with a similar issue and how they resolved it.
Hope this helps.
ETA: Also, if you've been pulling the file from your wife's Nook, maybe try re-downloading directly from her My Nook Library on the website. It could be that there's something extra added to files received on the devices via Wi-Fi/3G which is interfering. Assuming your wife doesn't sideload her stuff as a matter of course.