Okay - no other users are encountering problems like this? I know there are quite a few calendar synching problems with the BES and Outlook as well. When items are updated I've seen where they will not synch up properly.
Something changes, BlackBerry does not know what to do with the item and removes it. If he cancels it and attempts to create it again it will probably go through without problems. Only issue is will the user want to delete a meeting and recreate it?