

Debian Stable. Get it installed, get everything working right and configured the way this person likes it on a reasonable DE with default themes, and more likely than not, you won’t have to touch this thing for years.
The setup’s not necessarily for noobs, but if you’re the one doing the setup, you should be able to get it into a place where it will pretty much never break for them.
You should probably give them KDE or GNOME (probably KDE, as it’s more Windows-like and less my way or the highway than Gnome). As much as I love XFCE, it’s probably a good idea to give a layman a feature-heavy DE so that nothing is likely to be missing; also, it’s way too easy to accidentally delete panel items or entire panels on accident and a little annoying to restore things back to the way they were. KDE’s panels implementation mitigates these issues.
If you don’t like bog standard Debian, you might really like Debian Testing.
It allows you to get decently new packages; I’d say typical lag is one week to a couple months depending on the popularity and/or complexity of the project.
I’ve been using it on my desktop for over three years just fine. It’s been quite stable while still getting new software versions in a mostly timely fashion.
Do note though that Testing means Testing; it’s not really concerned with being a rolling release distro, but with preparing for the next release, so there’s a few quirks:
Personally, I’ve grown tired of Debian Testing and rolling release in general; while I still using Testing on my desktop, I’ve thrown Debian Stable on most things I’ve owned since then, and if I really need a newer version of software, I’ll just install the Flatpak or use a container.