The motherboard supports DDR3 Only, so it would be best to upgrade the whole motherboard. I'm pretty certain he has a prebuilt, so that'll be a little hard.
Not sure if he ever changed the CPU, I'm not faimiliar with the Z mobos so I don't really know about the whole K situation. I don't think he will change the cpu. What problems is there with that?
I have now suggested him to get more ram, 2x4gb DDR3.