Is file sharing on in your PC? What version of Outlook are you using, client or server based (and on what Exchange server)? What version of OS X is she using? Does your system administrator allow this?
Is there anywhere a Mac Office installed (for testing with that) in your office? I think Entourage should handle the proprietary formats of Outlook's Shared folders better than mail.app.
If the folders are shared directly from Outlook, and not from an Exchange Server, you're out of luck. Otherwise, Entourage is your only real bet - the most recent versions give you good access to Exchange Server public folders.