I see your edit, and would like to comment that you also haven’t mentioned which rpi you were going to try this on. I recently wanted to set up a quick little pc using a rpi, just so I could browse the web and maybe watch some videos. Tried a rpi2 I had laying around. No way. Moved to a rpi3, and while it would load most pages, youtube was a bust as it completely maxed out the ram and swap. I then went to a rpi4 with 8gb and it has been doable, kind of. Sometimes it’s pretty choppy with the video, but basic web browsing is ok. I did try my plex the other day through the browser and it was barely acceptable. Lots of stuttering and sometimes just hangs, but that’s not the ideal way to view plex on anything. Haven’t tried my rpi5 yet, but my point to all this is that I think you are on the right track setting it up on a spare pc he has. If you had given him a rpi as a ‘desktop’ replacement it might have just soured him on the whole idea.
Just as a general rule, I would start checking log files. You can start by searching /var/log for files that have been modified in the last few mins with something like “sudo find /var/log -mmin -10 -ls 2>/dev/null”. That will get you all log files in /var/log changed within the last 10 mins. Then you can tail those or grep them looking for clues. I have done searches of the entire file system looking for log files that were recently modified to find clues. It might also help to send the output to a file so you can view that and scroll up and down rather than just trying to read the output of the find, tail or grep commands. Put a “1>/{path}/filenameyouwanttouse.out” at the end of the command or you can pipe it to the tee command and it will show on the screen and write to the file you specify.