Hi Andrea,
Which method does he use to share the calendar?
Given this situation, please try share calendars on the web mail and see if there is any difference.
And there are some suggestions for you to troubleshoot the problem:
1. Start your Outlook in safe mode (Press Win + R, type “outlook /safe”, press Enter.) and see if the issue continues.
2. Repair your Office installation.
3. Create a new Outlook profile via Control Panel > Mail and test.
Any update, please feel free to post back.
Regards,
Aidan Wang
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