I looked at the standard greeter under FC3.
It looks like it simply polls the passwd file and removed root and other system accounts from the chooser option.
I seem to remember a richer version at some point though in FC line. Which had options for user pictures and other assortment of things. That does not seem to be the vanilla version I have.
Worse case scenario, remove and re-add her account (copying files to a temporary location of course). See if that does the trick.