I'm pretty sure the No'kaled is a agi weapon? So he shouldn't be using that.. at all. Also, why put "lol" after souldrinker? It is proven to be BiS for Frost DW. (at least, the heroic ones are, not 100% sure for normal, I'd go for the heroic axe from morchok before killing Madness HC).
Anyways, if he has 2 397 DW weapons (with str, not agi) then he should stay Frost. I play unholy atm with the Gurthalak (Normal version) since I've been unlucky with nice DW weapons so far. And since I do more DPS in Unholy now, thats the way I go. But DW Frost is still equally, if not better than Unholy.
Why are you lol'ing at the dk having Souldrinker, it's one of the best weapons for frost other than heroic HoM. There are also some interesting results on ej about using No'Kalad. The 390 Gurth is better than any 397 alternative, so with what he has now as dw frost they should be roughly on par. If he plays the specs at about the same skill level that is.
he said "normal' so yeah 403, sorry wasn't thinking about it.
As I said, an unholy dk with 390 gurth will do as much damage as a frost dk with 2x 397 weapons. However if I am understanding you, he has 2x 403 weapons as frost in which case he should probably stay dw until he gets the 403 version of gurth. Yes it's that damn good.