Sorry to hear about the trouble your colleague is facing. I would like to know if the steps suggested above by @Eugene Tyson worked for you? If not, I would request you to share a few more details like:
The exact version of InDesign
Is it possible to upgrade macOS to Big Sur and check if this is happening there as well?
Is the issue related to a specific document or all?
Are you using an external monitor(s)? If yes, with which port and cable combination it is connected.
What is the scaled resolution in Mac Display System Preferences? (Hover over the scaled resolution to see the screen resolution.
If you are on InDesign 2022, what is the UI Scaling in InDesign Preferences?
Screenshot of “About This Mac”. (both Overview and Displays tab)
We are here to help. We just need more information.