Actually, it will depend on what kind of editing he's doing. If you're into photoshop, then 16gigs might not even be enough if you do it extensively. I've managed to max out my ram in editing my videos, and I don't do all that much to them. So 32 might be a better bet if you're a huge editor.
Other specs to look at are how many cores the cpu you're looking at has. A lot of AMD CPUs will have 6-8 on a lot of the mid-high end cpus. Were as the intel ones will have better performance per core, thus 4 cores would be enough, again, depending on how heavy the editing is. If its heavy and you're set on intel, an i7 with hyperthreading would be your best bet. But if you choose AMD (which is what I have) I'd go for the 8320 and up. I have the 8320 and its amazing what this thing can get done. Just make sure you have a really good cooler, or else you're more than likely to overheat when rendering. So, basically, you're not looking at the brand of computer, so much as the brand of the components and the type of components within the computer. Building your own would for sure be your best bet.