That’s a tough one. Maybe have him send a brief e-mail to his regional admissions officer to express his strong interest in the school, and mention that he now realizes he made an error regarding the name of the building in his essay and that he hopes they will overlook that? In some cases contacting them means you’d just be calling attention to a mistake they might not notice, but this sounds like it’s going to be more obvious. Still, this sort of error does not constitute misrepresenting himself, so it’s not the end of the world in that sense.