Just because you can doesn't mean you should
As Kaamos said, I strongly recommend a fresh install of Windows. The longer he's used the current system, the more software he's installed and uninstalled, the more 'careless' he's been with where he gets stuff, etc etc—just dump the dreck and start fresh.
I hear what your saying. We just did a fresh install on his current hard drive not too long ago because his other one was failing. But I will work on getting him a SDD and well do a clean install when I get it.
That will do fine for storing data, again as Kaamos said—can also store games from Steam in a pinch.
You haven't said what kind of games he likes, or where he buys them. If he buys a lot of big AAA games from Steam, then a 1-2TB NVMe is best so he can keep a few installed at a time—altho that's not a biggie if you have good internet download speeds, which enables frequent install and uninstall as needed.
He buys all of his games from steam, I am really not sure what games he plays, I know he mentioned God of War, but he plays alot of VR games as well. Internet speeds are pretty consistent at around 345mbps download.
You can move Steam installs from one drive to another, without any uninstall-install loop. So you could keep a bunch installed on a big cheap HDD, and move the current one(s) to the NVMe, and off again later.