Clutch cables are $12 and easy to install.
If he hasn't put one on it to fix the problem the quad better be SUPER cheap.
Clutching and shifting are two totally different things.
You can visually inspect the cable easily.
Anything else will be internal. Or at least under the clutch cover.
Heck, if the cable IS bad, and the swap is that good and you want it, If I were you, I'd buy a cable and install it so you can ride it to check everything else.