My 2c worth.
Outlook recurring events ties all events in the recurring group together. when you delete a recurring event in Outlook it asks if you want to remove that event only or the entire recurring group.
I would suspect outlook doesn't remove the recurring event on the day it has been removed in all the other devices as it doesn't know to remove just one instance or all instances.
Potentially a recurring event isn't actually saved as a calendar entry in Outlook but as a dynamically populating entry - that is shown every time a calendar is opened.
To resolve the issue get the user to delete via outlook - see if that works.