Usually it happens when they have too many listed as favorites.
If it isn't that, and they are just trying to have a few favorites and can't. Then export the addressbook off the machine to their desktop and delete them from the machine completely, then import the exported file.
yes, seen that issue too
in a specific firmware of the 78xx models, the book gets corrupted
you can export the addressbook, manual edit the file , set the favorite flag on, then import again
then you have the favorite
or delete all addressbooks first, then upgrade the machine to latest firmware, and start editing again...