I strongly recommend going up to at least a microATX case. what you can get for $500 in mini-ITX is severely limited, while you can build a strong gaming machine on $500 with micro ATX/ATX.
If he absolutely refuses, we need the make/model of his case. This is pretty critical, as we need to know clearance inside the case. Whether it's a cube or a slimline tower and the inside measurements affects what video card will fit (if any).