Outlook Calendar Share from 2010 to 2007

Someone in my office running Outlook 2010 is trying to share his calendar with 2 others running Oulook 2007. We use Exchange Server 2010. I set up the permissions on the 2010 calendar for each of the other 2007 users as Reviewer on the 2010 user's machine. I have read this is enough permission needed for someone running Outlook 2007 to view the calendar. However, when I use Open a Shared Calendar in the 2007 machines, I get the error "You do not have sufficient permission to open the Calendar folder. You don't have appropriate permission to perform this operation."

I've Googled it, but all I read is people changing the calendar permission to Reviewer to solve the issue. Is there something server-side that can be or needs to be changed? I believe that one of the 2007 users used to have this working, but since the 2010 user (the boss) changed his privacy settings on the server, it stopped working.