A quality 600W will run his system easily.
That is true, but if you compare the prices of a AX 650 and a AX 850, there is just a 40€ price difference. And since he already has some quite expensive hardware, I don't think that he wants to get cheap with the PSU.
If he gets a second graphics card, 600 watts might not be enough. Or it might, but just and just.