Whatever you use, so when he asks for support, you're in a position to help

likely You'll be supporting them, so whatever you're more comfortable trouble shooting

I would recommend Linux Mint. Its really simple to work with and basically plug and play. Comes with a solid software selection and built in tools. I personally really like the cinammon and mate editions. Cinnamon has a bit more modern look to it and MATE is a bit more retro. 

A lot of people are going to recommend you mint, I honestly think mint is an outdated suggestion for beginners, I think immutability is extremely important for someone who is just starting out, as well as starting on KDE since it’s by far the most developed DE that isn’t gnome and their… design decisions are unfortunate for people coming from windows.

I don’t think we should be recommending mint to beginners anymore, if mint makes an immutable, up to date KDE distro, that’ll change, but until then, I think bazzite or aurora if you don't like gaming is objectively a better starting place for beginners.

The mere fact that bazzite and other immutables generate a new system for you on update and let you switch between and rollback automatically is enough for me to say it’s better, but it also has more up to date software, and tons of guides (fedora is one of the most popular distros, and bazzite is essentially identical except with some QoL upgrades).

How common is the story of “I was new to linux and completely broke it”? that’s not a good user experience for someone who’s just starting, it’s intimidating, scary, and I just don’t think it’s the best in the modern era. There’s something to be said about learning from these mistakes, but bazzite essentially makes these mistakes impossible.

Furthermore because of the way bazzite works, package management is completely graphical and requires essentially no intervention on the users part, flathub and immutability pair excellently for this reason.

Cinnamon (the default mint environment) doesn’t and won’t support HDR, the security/performance improvements from wayland, mixed refresh rate displays, mixed DPI displays, fractional scaling, and many other things for a very very long time if at all. I don’t understand the usecase for cinnamon tbh, xfce is great if you need performance but don’t want to make major sacrifices, lxqt is great if you need A LOT of performance, cinnamon isn’t particularly performant and just a strictly worse version of kde in my eyes from the perspective of a beginner, anyway.

I have 15 years of linux experience and am willing to infinitely troubleshoot if you add me on matrix.

Fedora if he's not gaming.

Bazzite if he's gaming. Or CachyOS.

I'll give you the secret to easy linux: stick with defaults! Stick with distros aimed at whatever you're tying to do, and you get a whole army of very experienced developers preconfiguring it all for you, for free. Instead of having to maintain breakage youself.

For example, do you want to learn all about debugging AMD drivers? Do you want to get into the intricacies of performant Proton setups, and environment variables, and kernels stuff?

You could just not, and get all that prepackaged!

Here's just a sampling of some pre-configured stuff in my distro:

cachyos/protonplus 0.5.14-1
    A simple Wine and Proton-based compatiblity tools manager for GNOME
cachyos/protontricks 1.13.1-1
    Run Winetricks commands for Steam Play/Proton games among other common Wine features
cachyos/protonup-qt 2.14.0-1
    Install and manage Proton-GE and Luxtorpeda for Steam and Wine-GE for Lutris
cachyos/umu-launcher 1.3.0-2
    This is the Unified Launcher for Windows Games on Linux, to run Proton with fixes outside of Steam
cachyos/vkd3d-proton-mingw-git 3.0.r0.g6d97b022-1
    Fork of VKD3D. Development branches for Protons Direct3D 12 implementation

cachyos-znver4/mesa-git 26.0.0_devel.216300.02cfc61cc93-1
    an open-source implementation of the OpenGL specification, git version

Do I know a thing about how Proton works? Nope. Do I know anything about maintaining an upstream AMD driver for some kind of bug fix? Absolutely not. And I don't have to! It's just there, in sync with the rest of my system through some maintainer's magic.

Fedora is perfect for beginners. Stay away from immutable anything if you're going to be helping support this.

No matter what you end up using, make sure he installs RustDesk, so you can easily connect and fix the inevitable problems

Bazzite works pretty well out of the box and with new hardware (n = 1)

Depends. Why does he want to use Linux?

For new users, I've been recommending Zorin with good success.

Based on what I've heard about Bazzite I'd pick it over Fedora for your friend, but I've always used Debian-based distros so I can't speak from experience.

My PC is the same build and I run Kinoite (also KDE). The only major issue I ran into was figuring out how to get a Windows VM up and running for work stuff, but I eventually got it and now that's working smoothly, too.

The majority of what most people run is available as a Flatpak container, but there's also rpm-ostree if you want to install packages, which functions similarly to dnf. And you still have rpm if you want to install something manually.

As far as AMD driver support goes, everything's been working great. Can't say how it compares to base Fedora, though, but it's probably similar.

I’m using OST right now and I like it’s features quite a lot

Is your friend a beginner? If so, Mint or Pop_OS! should be considered, since they're both Ubuntu-based. Bazzite is not recommended due to its atomic nature being kinda wonky, and Fedora I wouldn't recommend due to IBM being a bunch of stubborn bums about antiquated technology (i.e. X11) that works fine, compared to newer stuff (i.e. Wayland) which is not ready for prime time at all.

If he isn't, and knows what he's doing, CachyOS is a good choice, but I'd recommend he use Cinnamon as his DE, since it looks similar to Windows Vista or Windows 7. It does have both X11 and Wayland, but it's X11 by default.