You don't have to install it if you don't want.
Lots of people actually just use a live instance off a USB or maybe a DVD - though the USB is much faster these days.
If you do download Windows 10, make sure you get it from Microsoft. So long as he's still got the sticker on the back (assuming the laptop came with 10) then he can reinstall it.
This looks promising:
https://www.microsoft.com/en-us/software-download/windows10ISO
I've got several versions of windows 10 here on flash drives. Hell, I have a copy of it that installs all the software for a certain company I won't name.
LOL You might as well use the most recent ISO that has all the updates included. That appears to be from October.
I don't know much about Windows, but that seems to make sense. That way at least you know you won't get fucked over by an update, or have troubles because of one. Well, at least it lessens the odds of that happening.
I am going to try to boot this laptop from the flash drive and see what its like
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