It really depends on the type of riding he enjoys. I'm 6'4ish with a fairly long body/shortish legs. I found my xl Stumpy FSR a bit too big on more technical stuff, and would probably have been better off with a Large. That said, my large Pitch felt a bit small on anything that wasn't fairly gnarly. I now ride a 19ins hardtail, which fits perfectly.
I'm not sure I buy into the whole "tall people should ride 29ers" thing either. 29ers suit some people and their riding whether short or tall. Just becuase you're tall doesn't mean you'll necessarily enjoy riding one more – it might just be that they're more aesthetically pleasing than a 26ins gate.
The moral of the story – make sure he tries them out properly on the sort of terrain he usually rides on before he buys, or buys second hand so that he doesn't lose much money if it doesn't fit.