He's already putting a 4690K in his build, so he's not really skimping on the CPU, but assuming that changes in development don't reduce the dependance on the CPU, he could get a cheaper GPU. Remember DayZ only in alpha and it's moving to a new engine. This will possibly reduce the dependance CPU as one of the current issues is that the Zombies are full soldier builds and this puts a lot of strain on the CPU. If they are moving to a new engine, I see this as a likely change, because without it they cannot add more zombies without denying a lot of people entry to the game.
Also I'm assuming that while DayZ may be one of the main games he wants to play, he will probably want to play some other games, not all of which will be as CPU intensive as DayZ, so he will see benefits from a better GPU.
But if he is only playing DayZ, then yes he could get a cheaper GPU. Even an R9 270, paired with a good CPU, will play DayZ on high with decent fps in the wild, and maybe a few tweaks to the settings when in the cities, especially if they improve optimisation which is currently pretty rubbish.