Am4 is easily upgraded. 5600x is best bang for your buck. 16 is low but usable. With memory prices I'd wait. Video card...1660 is old sauce and the 590 driver is end of life. Newegg has a whole section dedicated to used cards. You can get a modern 3060ti for around 250ish.
If your gonna ditch win 10 for Linux, I'd recommend getting an AMD video card instead. Otherwise rest fits.
He might not have a board that can upgrade to a 5000 series CPU. Older boards needed a BIOS update and not all of them had enough storage for the update. Most of them did (some by completely overwriting the previous BIOS) but not all were capable.
Either way he's sinking money into old parts. For his use case, he's better off swapping completely.
With a CPU, RAM and storage upgrade, this would make a great little home server though.
Depends what his budget is, frankly. Your right about the 5000 bios, but even a 3600x would be a sizeable upgrade.
A new rig with parts that can play modern games will run around 800-1000 on the lower sale end. A 3600x and an 3060ti can be got for around 350.
I can't tell anyone what to do with their use case. Just giving him options