My brother bought a 2000 YZ250 and I had a chance to ride it for a few open desert and trail miles and my hands felt like they wanted to vibrate off my wrists. I had fun riding a light bike compared to my WR450, but you get exhausted riding with so much hand vibration. Is there something I can get him that will lessen the hand vibration. Maybe different triple clamps, or rubber fittings, or whatever.
Something is not right if it is vibrating. Your crank might be out of balance, which is the only thing I can think of.
I had the same issue with my KX500, which is a diffrent issue. But I was able to cure most of the hand vibes with Pro Tapers and filling it with expanding foam. Also, try some anti vibe grips.
I would look into maybe the crank being out of balance or some motormounts broken. Good luck.