How to remove a standard user from iMac without their Apple ID password?

The iMac I use at work has a standard user, belonging to an ex-employee. This person left the company so I would like to remove the user account from the Mac. I have an admin account. The problem is that I can not remove the user without their Apple ID password because of the Find my Mac feature. I have contacted the ex-employee who deleted the Mac from their iCloud using an iPhone - proved with a screenshot. But I still not able to delete the user, macOS constantly asking for the AppleID password. I have tried rebooting etc - nothing works. What else I have to do to remove the account? I have tried factory reset the Mac, but that's not possible without the password either. I can contact the ex-employee if further actions needed, but obviously I can not ask for the password...