Buildapc

There's a great forum for your friend!
https://linuxmusicians.com/viewtopic.php?t=27775

My understanding is that the Focusrite USB-C cards works as expected in Linux but you want a third party GUI such as:
https://github.com/geoffreybennett/alsa-scarlett-gui

Case: I'd go for a case with sound dampening and dust filter, a cheap mATX one would be Chieftec Pro Mini
PSU: I always want a modular PSU and I always aim at one with at least 80+ Gold. A quiet one would be the Seasonic Core GX V2 650W ATX 3.1. Can't go wrong with Seasonic.
CPU: AMD Ryzen 5 9600 is low TDP, cheap, latest gen AM5 and should be enough for Reaper without problem.
Motherboard: I went for a cheap B850 board but verified that the PCI Express slot had PCIe 5.0. An example would be ASUS Prime B850M-K.
Drive: An M.2 NVMe Gen 4 2TB should be enough to store both sounds/instruments and the reaper projects.
RAM: I went for 2x16GB DDR5 6000MHz CL30 with AMD EXPO. If your friend knows he uses abnormally large sample libraries it doesn't cost all that much to increase it to 2x32GB but should be unnecessary.

My total cost ended up ~€840 with that.

Definitely doable, but my suggestion would be to not underestimate the demands of audio processing. It's heavily CPU bound. Recording, mixing, playback, etc. Also, give it plenty of RAM so the DAWs can breathe and run more easily. It starts with recording but plugins, virtual instruments, digital synthesizers, effects, EQs, compressors, over dozens or hundreds of audio tracks; eventually it all taxes the system as much as graphics editing would do minus the graphics card. You still need a decent machine to avoid lag and hang ups. I used to compose and mix on an old cheap ASUS laptop, it did it, but I wouldn't say the experience was anywhere decent. It was slow, it was frustrating and it was riddled with delays and problems. Don't skimp on those two areas, CPU and RAM. Everything else can be cheap, maybe you can do with an integrated graphics CPU, like a modern mid-range Ryzen (anything above the Ryzen5 7500 should do) or something equivalent.

That said, Linux still struggles with some aspects of music production. I did it as a hobby about 5 years ago, but it was very technically involved. Almost no common distribution's kernel is properly optimized for low-latency audio, so custom kernels are the norm. The compatibility of software and plugins is appalling and the compatibility layers like WINE to run Windows only plugins or software require some gnarly hacks to get it to work. You should get familiar with the technical intimacies of pulseaudio, pipewire, alsa and jack as soon as possible, they are the backbone of audio on Linux and so they are the biggest hurdles as well. Perhaps prefer to use some distro like AVLinux or Fedora Jam instead to deal with these problems out of the gate. So, be transparent about limitations, adjust expectations, do something creative with them, the obstacle is the way when we talk about creativity.