You can't. Remote Desktop on Windows 10 Pro does not allow using computer simultaneously. When you connect with Remote Desktop, the active session will be “disconnected”. If your friend switches to his account, your remote session will be disconnected.
Only Windows Server allows several Remote Desktop sessions simultaneously.
Even though your user account session is not active, all the open processes will continue to run in the background. When you connect with Remote Desktop later, you'll see your open apps.
So you can start your rendering work, and then allow your friend to use his computer. You can connect occasionally to track the progress, Remote Desktop will warn you if there's an active session.
2You can use rdpwrapper but technically not allowed under microsoft terms. Only one user is allowed on Windows10 pro at a time. For multiple users, he would need to be running windows server. – Narzard – 2018-03-11T02:38:17.803
I don't know. But i'll just mention a few things. Remote desktop(when i've used it at least), would log them off.. vnc would share screen, both those would be a disturbance(though the latter is much less of a disturbance). Accessing command line won't help 'cos presumably you want to use the GUI. There are applications like GoScreen but AFAIK they aren't built to work with VNC.. With VNC on linux you can go to different desktops. – barlop – 2018-03-11T02:44:23.300
1Hmm. "Heavy render task" and "I don't want to disturb him" don't really go together. I think his computer slowing down might disturb him ... – DavidPostill – 2018-03-11T08:59:39.397