If they are running Windows Pro they could use Remote Desktop. You can configure the connection via port forwarding or have them VPN into the office network from home and then do the remote desktop.
If not running Windows there is software on the Linux side that you can install on the computer for remote access, but that is beyond my knowledge. Remote access setup would be similar though (port forward or VPN). I prefer VPN myself, but either works with strong password and good TLS.