You shouldn't add from your Store / purchased section. Add from your Library section (music, movies etc.). If audiobooks isn't showing here, go into the iTune menu at the top, clicke preferences, General. You should then see all the media types you have selected to show. Click Audiobooks so that it is ticked, close preferences. You should now see Audiobooks under the library section on the main iTunes screen. You can now add them from here to a new playlist like you would any tune, then sync to your Mum's ipod.