Open Other users folder in Outlook 2008

I have set the permissions for one of my users to open another users inbox. Whenever I try to open it I get the error: Unable to open folder. Folder could not be found.

I use Outlook 2003 and when I set myself with the same permissions it opens without any problems. The user is using Outlook 2008. Any ideas why she can't open the folder and I can.

Thanks