Rationale :
You're sure you don't want to wait for some DX12 cards to release? They're supposed to in just a few months. It'd suck to buy an epic card like an R9 280X or GTX 770 now, and then miss DX12 immediately.
Depends on how bad he needs a GPU. If he doesn't have anything that will play decent games
then it might be worthwhile just to buy a DX11 one now.
I am personally skeptical about DX12. Unless it is backported to 7 it will not get much adoption