I had considered putting Xander in this position since he's the one I write much more often, but Xander would say something that would make Snape turn him into a toad for sure. No way could Xander keep his cool. Riley, on the other hand, is a trained soldier who the government throws at any weirdness that pops up. Yep, he's got the discipline to prove to Snape that not all muggles are so easily dismissed.