A Micro ATX board will fit into just about any size case. Even full towers have screw holes for you to be able to attach smaller motherboards. That's part of the beauty of a full tower, not to mention the air flow. If he has a bigger case from another computer lying around at home, don't even bother getting another slim case. Just move the board and the components to the bigger case. And if you can find a bigger case, then don't even bother with slim graphics cards anymore either. A bigger case opens up the bigger and better options. Generally the slim computer and slim video cards are designed with Business uses in mind. They save space, and use smaller and less expensive components to be efficient and save space and money.
Most of the slim video cards will not be over-the-top performers purely for the Business reason. The other reason is that they're made on a smaller die, and they can fit less onto a smaller chip. That means fewer stream processors, less space for RAM chips, and that means less performance.