If it is possible to open Outlook, make a backup.pst file by going to File -> Open & Export -> Import/Export -> Export to a file -> Outlook Data File (.pst)
This will create a file in the users Documents\Outlook Files\ directory, and you will be able to recover any emails that have not synced to the server. This is necessary because you CANNOT recover files from an .ost file.
Completely Re-create their mail profile
1. Log in as yourself and make sure the user is logged out.
2. Make sure Outlook is closed, and navigate to C:\Users\userinquestion\AppData\Local\Microsoft (You need to enable Hidden Folders in File Explorer)
3. Rename the folder "Outlook" to "Outlook.old" or whatever you want. (This is just in case we need to recover data)
4. Navigate to C:\Users\userinquestion\AppData\Roaming\Microsoft
5. Rename the folder "Outlook" to "Outlook.old"
6. Login as the user in question. (I have been using TeamViewer)
7. Open Control Panel
8. Search for Mail and open it.
9. Click on "Show Profiles... "
10. Remove all profiles.
11. Click "Add..."
12. Name it whatever you want, the original name is "Outlook"
13. The user's data should be autofilled.
14. Click next. The profile should be created.
15. Open Outlook. There should NOT be a Duo prompt, and the user should be able to use the application again.
*Instead of doing steps 7 - 15, you can just re-open Outlook, and the profile should be re-created.