I recommend avoiding Windows 10. Not only do you bypass
Microsoft's built-in spyware, you (inadvertently) bypass a lot of malware simply because nobody fishes in an empty lake. All the malware boogeymen flock to
the most commonly used operating systems, not older ones that few people use. Is it secure? Not really. But if you're truly paranoid,
security essentials will cover older systems from generic badware without much exception. My personal recommendation for a normalfag OS is the earliest version of Windows 7 Enterprise/Ultimate. (I won't provide a link to that, Null has enough headaches.)
From there it's figuring out what your friend's parents want to use. I assume they browse the internet, so I recommend setting their browser to some decent
Chromium derivative and plug in
Adblock,
uBlock,
Adguard,
Scriptsafe (old people may get annoyed at having to manually 'okay' scripts, but it's worth it for security) and
Minerblock. I've clicked on web pages I knew were infected with these add-ons active and nothing made it through.
After that it's the usual promenade of installing drivers for whatever they use.
Run this and
get USB support if you decided to go with what I recommended.