Response by Brad Dinerman
First, I’ll assume that this is a public folder calendar rather than a mailbox calendar that has been shared. Given that assumption, then this could be a permissions issue. Go into Exchange System Manager (ESM) and look at the properties of the public folder. Verify that the user that cannot see the appointment has the Read permission set for that calendar.
If that was not the problem, then also check whether the user is running Outlook in cached or offline mode. Although I suspect that is not the problem, I’ve seen many unusual things occur with respect to these modes, so try turning off cached/offline modes if possible to see if that resolves the problem.