Executive assistant making a calendar entry wants to mark it private

I have an executive assistant who uses a secondary (added in) mailbox for the exec in her Outlook configuration.  She has full access rights as far as I can tell (security and Exchenge rights), but is logged in as herself and accessing the exec's calendar through her Outlook profile.  If she creates a calendar entry (which works fine in all other cases) for the exec, the PRIVATE button is greyed out and she cannot set the entry as private.  Can this be changed or it it a limitation of the Outlook / Exchange setup in this configuration?  Outlook 2010, Exchange 2010.