Q:

Use Ubuntu 18.04 laptop as second monitor for Windows 10

(TL;DR below)
My girlfriend is a teacher and needs to use Windows to use some online teaching tools provided by the government of the country we live in, that do not work properly on Linux (I tried with several Ubuntu flavours and versions).
On my laptop that mounts Windows, the screen had to be changed and the cheap replacement I bought was not supposed to be of good quality, and now that she has to spend many hours in front of it, she really feels it on her eyes.
Her laptop, on the contrary, mounts Ubuntu only (was shipped with freeDos) and its screen is still rather good.
TL;DR: how can I use the Ubuntu 18.04 laptop screen as an extension of the Windows 10 desktop?
I know that I could do it if both laptops used the same OS, but I did not entirely choose to be in this situation :P

A:

Provided both computers are in the same network and as you do not want to use both monitors at the same time, you could use an rdp (remote desktop protocol) client on Ubuntu to access windows.
Unfortunately, I can not provide you with the windows side of things (but it is an internet search away). On Ubuntu you would use e.g. remmina.
PS1: Rdp is totally usable for such scenarios. During lockdown we use it on linux laptops via ssh tunnel at work to access windows desktops for people who need them.
PS2: I understand that this does not literally answer your question, as you refer to a second monitor, but it would seem to me to solve part of your problem.

