Can't see your email addy in your profile C_G, so I'm posting on here.
I'm 6'4" with a 35" inside leg. I have several bikes, a Heckler, a on-onme 29er, and a 1997 Litespeed. They're all XL, 20"+, but the geometries of all of them is different, my position on all of them is very different, and (in the case of the litespeed in particular) has changed as my riding has progressed.
I'd go for a 26" bike for more rocky, technical riding (not that I'm any good at it), and a 29er for longer distance "epic" old school XC.
Personally, I'd make sure that the TT is 24" plus - this way he can run a short stem if he wants to without feeling too cramped.
My main feeling though is that he's going to probably ride it for a while and then tweak it anyhow based on his preferences and the stuff he's riding, so as long as the frame is a reasonable size he can fiddle with position, stem etc later on.
I think you're looking at 20 or 21" frames, personally (I've avoided Giant, GT and Trek 22" frames in the past as they seemed much too big); the main thing for me is length!