If the user is using Windows 10, make sure the following has been applied:
Cumulative update for Windows 10 Version 1607 and Windows Server 2016: November 8, 2016
We have a user that experienced this same issue. They installed the Grammarly.com plugin to Word which was the culprit. I've logged this issue with Support, but the workaround here was to uninstall Grammarly.