You know that any Contacts folder in their primary mailbox can be made into an address book using the Outlook Address Book tab of the folder properties. An address book appears in the drop-down list in your customer’s Outlook client:

…but there is no corresponding contacts folder in the Contacts view:

Your mission, should you choose to accept it, is to determine the source of this mysterious address book and make it visible in Outlook’s Contacts view.

Fortunately, it’s not impossible, just obscure. There are two possibilities.

1. A contacts folder has been listed as an address book with a name that is different than the folder name. In this case, all you have to do is go to the Outlook Address Book tab of the folder properties and change the address book name.
2. At some point in the unknowable past, the user accidentally deleted this address book (or dragged it into the trash), but it hasn’t been purged from the Deleted Items folder. You won’t be able to see it in the default Mail view. You’ll have to put Outlook into the Folder view using the little folder icon at the very bottom of the navigation pane. (In Outlook 2013, the folder icon might be missing. Instead click on the ellipsis–the three little dots like this: …–in approximately the same location to get to Folder view.) This too is an easy fix. Just drag-and-drop the deleted folder back out of the Deleted Items. The default Contacts folder would probably be a good place for it.

This works for Outlook 2010 and Outlook 2013. It’s probably the same for Outlook 2007.