Have you watched him ride? Sounds a bit to me like he's lugging the bike too much, like he is a gear too high and the engine is loaded too much. Is it his first modern single cylinder? He may be used to older big bore singles which love to punt along at low revs, whereas newer 450s need to be ridden in mid to high revs. What gearing is he running? Just trying to rule out operator error, there are plenty of times where the problem is between the handlebars and the footpegs!
If not, I'd say what you've already suggested - wheel balance, check for runout/buckling, and steering head bearings.
Have you ridden the bike and experienced the problem? He may not be used to the different riding style and power curve of a single cylinder
2006 DR650 tard : 725cc, Waggott Cam, FCR39, Twin Air, dual CRF250R zorst, RAD hubs/Behr Rims, 16/45, EBC 320
2006 DRZ400SM tard : 398cc, FCR39 slant style, Twin Air, Micron slip-on zorst, 15/41, standard otherwise ... still crazy fun (gone)
2004 'Berg FS650E : 644cc, stock, work in progress to fix main bearing failure