Sharing calendar problem for users with different version of Office installed.

I can’t understand what’s happening in our Exchange 2003 network. We have many users in our network. But user 1 can not share user 2’s calendar. I mean user 1 can share all other’s calendar and user 2’s calendar can be shared by all other users. All Users have Office 2003 except user 2 has Office 2007. Is there any solution?
