Where does it show the mailbox is full?
If you're talking about the web panel, then two things could come into play:
1) The stats are updated somewhere after midnight local time. They are not real time. Maybe there was 16MB of email and she has since downloaded her mail.
2) The storage stats you see there is for the user. If the user is also a 'real' user and not only a mail user, then she could have 16MB worth of files in her userpace on the server. i.e. files she has uploaded/accumalted in her web and private space.
Hope this helps.