By default you need to have full mailbox access to open a co-workers mailbox.
To open a shared calendar to which you have been given rights by your co-worker, do the following:
append /owa/full_email_address_co_worker/?cmd=contents&module=calendar to the url of Exchange 2007 webaccess.
https://webmail.domain.com/owa/name@domain.com/?cmd=contents&module=calendar
This should also work for Inbox, Tasks and Contacts:
/?cmd=contents&module=inbox
/?cmd=contents&module=tasks
/?cmd=contents&module=contacts
source: Microsoft Technet