The one thing you might want to think about chanching is that you get a small motherboard and case. Unless there is a need for more graphics cards later on, why not get a micro ATX board and a smaller case?

Agree, mATX tends to be cheaper without less features than an ATX board, and fits in more variety of cases.

If he gets an RX 480 with reference blower cooler design, I recommend getting a motherboard with only 1 PCI slot between the 2 full size GPU slots, which lets you choose a smaller case. If he goes for an after market cooler on the RX 480, I advise two slots between the full size slots for better air flow, and a case with at least 5 PCIe brackets on the back, or preferrably an ATX case with bottom fan mounts.

If in the future he wants to expand to a second card (which is a nice option to have no matter what he plans) that's what I'd recommend.