I want to quickly augment to my last post.
As an admin, if you want to find out if the client indeed created a message in the appropriate free/busy folder, then go to https:///public/Non_IPM_Subtree/, navigate to SCHEDULE+FREE BUSY select the appropriate administrative group folder. That path can be derived from the mailbox's legacyExchangeDN attribute and do a search for /User-/CN=RECIPIENT/CN=, also derived from the legacyExchangeDN.
if you find just one entry, then everything is good. In case you find an entry with "-2" suffix, then it's time to delete both messages in that folder, and re-run Outlook for that user with "/cleanfreebusy".