Outlook Express address book, first names disappeared

I have a client who still uses OE6 for his pop emails.
For some reason SOME of the first names in his address book disappeared. That wouldn't be so bad except when he's looking for a family member he cannot tell which is which because of this problem.
The first names still show up if we double click on a contact and look at the Display field. But the First Name field is now empty.
Any idea how this might have happened? Or he would be happy if the address list were to show Display field rather than last name, first name, but I couldn't see how to change the columns.
thanks.