I would verify that their Exchange account is the only account within outlook. From a few different places it looks like it could be something stuck in Drafts too (although you said you checked there). Possibly removing the account and re-adding it in Outlook could be the quick fix to it (although they would lose their saved autocompletion which a lot of people complain when they lose it).
Outlook 2016 For Mac Standalone
Outlook 2016 hangs pretty much every day on the synchronizing step. It appears to be only in the IMAP accounts, but it can hang on any of the folders within the IMAP account. I am ASSUMING that Outlook on my PC is trying to sync with the server, which is in turn trying to coordinate the sync between the PC and the phone. Feb 27, 2020 Cannot send Calendar Invitations on behalf of Public folder in Outlook 2016/2019 for Mac. Outlook 2016/2019 for Mac does not support allowing subscribed Public Folder users to send on behalf of that Public Folder calendar. Users can send meeting invitations from their own Exchange account and can include that Public Folder as a recipient.
Sync Errors In Outlook 2013
Im running 2016 on probably 25 Mac's and haven't seen this issue yet, but I have had weird issues with Outlook in general that aren't worth the time messing with and just adding the account back again will fix saving all of our times.