What you've confirmed by looking at PR_IPM_APPOINTMENT_ENTRYID is that it is definitely wrong on that user's mailbox. It's very unclear how you would ever be able to access this user's calendar folder using Outlook. Here's one more command line switch that might reset the property: \CleanFreeBusy. Keep in mind that the owner of the target mailbox is the one that needs to run this, as their mailbox is the one which is broken.
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