Personally, I haven't done heroic garrosh, so don't put too much stock in my words. BUT, after looking over the warcraftlogs for frost DKs on 10-man heroic garrosh, they are literally ALL DW frost, not 2h. Since this fight has lots of adds, I could see the extra cleave damage providing huge advantage (since DW spams howling blast, whereas 2H prioritizes obliterate). On more single-target fights, I believe that the two play styles are much more comparable. However, on second skimming of some of these meters, it looks like DW is the dominantly used play style among top DPS DKs by and large. At max gear levels, they may pull farther apart.
So I'd suggest trying to get the guy some 1-handers. He's probably low even for 2H, but DW is easier to play anyway--if it's a player error issue, it should become even smaller as DW.