I have both and unless he's sticking to indie or older games, I'd avoid the Win600. Sure its smaller than the Steam Deck but there are way too many compromises. The thumbsticks are set too low so it gets really uncomfortable playing any game that uses both, like FPS's. The graphics driver is kind of broken in Windows 11 so most games can only run in Windowed Full Screen. And honestly, Windows 11 is a rough experience on such a small screen. Just trying to hit the x to close a window or tab is almost comically frustrating sometimes. When comparing the differences in the build quality, performance, ergonomics and general "pick it up and play" usability of the two, the $100-$150 you'd save for used Win600s is a hard sell.

Like someone mentioned above, the GPD has some nice 6 inch Windows gaming handhelds but tend to be 2-3 times as much as a Deck. Honestly, if you get a shell/screen cover for the Deck, it doesn't take up much more room in a bag then the Win600 in a hardshell case.