First off, is he sure he is selecting the correct file?
Second, is the pst his main mailbox that he wants to use to receive mail in? I.e. is he trying to get OL2003 to use this pst as the file in which he receives his mail?
Normally, I simply do:
File, Open, Outlook Data File and browse to the location of the file.
I have a pst that has travelled with me since OL2000 and I have never had a problem getting this to be my main mailbox - with no other pst in Outlook. What I do in this case is:
Create a new mail profile
This generates a pst in c:\documents and settings\my profile\local settings\application data\microsoft\outlook
Once I have successfully set up this mail profile with the generated pst (which has only the default folders - and that's what I think he is looking at), I then use Windows Explorer and navigate to the location of the pst that Outlook created, and I delete it.
When next opening Outlook I get an error that Outlook could not open the pst (obviously) and as a result, Outlook opens a "Browse" box for me to locate it myself.
At that point, I browse to my original pst - and it all comes back...
If it is password protected, you have to enter the password twice, but that's only the first time the pst is re-opened. After that, it is "normal"
Hope this helps.