I am pretty sure audio books are treated like music files. You can learn how to use the music player by checking your instruction manual on your Nook or look at the online version here:
http://www.barnesandnoble.com/u/Support-NOOK-Color/379003188 Look for the .pdf link for "user manual"
Page 119 is the start of the music app instructions.
There are also the special books that have read to me features but I think those are mostly for kids.
My understanding was that earlier models of eINk Kindles had Read to Me but that Kindle Fire didn't and that the latest eInk Kindle does not.
Is your girlfriend using some special app to get Read To Me feature? Or has Amazon changed something recently?