Basically he doesn't bother updating his computer, so I want something that will auto-update to fairly modern versions, that's why I mentioned stability as a requirement.

What's wrong with rolling release?

I would say rather "are regular updates really necessary"? Personally, I'm not sure they are (for most users). To paraphrase Debian, there's a lot of value in stability.

Manjaro is a mess. EndeavourOS is similar (both are Arch derivatives) but less breaky and something I'd recommend more.