This is my guess: (Don't hold me liable if I am wrong)
If you put it together negligently, and because of that something goes wrong and he gets hurt, you are probably at fault. Eg. the gas tank flips over and gas leaks on the engine and explodes...
If you put it together according to specs, and he crashes it because he is riding it unsafely, or it is another driver's fault, you probably won't be held liable.
Honda makes motorcycles... if someone buys one and then crashes it, is Honda liable? If you install a super-engine into your buddies musclecar, and he drives it too fast and crashes, are you at fault? What if a shop did it?
If anything, I would think the guy who sold him the kit would be liable.
Bottom line is that if he gets hurt and it is because of something you did wrong, it might be considered your fault in the court of law.
You might want to contact your attorney.
Good question though.