For a no-nonsense working system familiar to Windows users, it's hard to go wrong with Linux Mint, Cinnamon edition. If it really feels sluggish under that weight, maybe try out the Mate and XFCE editions, in turn.
Linux
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My vote is Debian with the Cinnamon desktop. Enable Flatpak and your friend should be good to go.
I would suggest Linux Mint. It is aimed at beginners, has a desktop environment that is relatively similar to Windows, and is based on Ubuntu (and therefore Debian).
1, It's a good idea to give them Linux if they agreed to it and have a general understanding of what is different.
2, I installed Debian on my 81 years old friend's shitty old laptop (it was out of curiousity), he never called me back to fix his OS. For your friend though, I'd recommend Linux Mint Debian Edition, as it feels faster than regular Mint.
Ubuntu or Mint are among the most noob-friendly.
But probably the biggest impact will be whether you go with Gnome or KDE. KDE is more Windows-like so could be a softer landing.
I've read a lot of stories where installing Linux resulted in less support calls, not more. It depends on how ambitious the user is - if they're mostly just staying in their lane and browsing the web it should be rock solid.