It's hard to say without having been there. I've definitely been in your shoes before, where I have to pull teeth to get someone to sit down and actually do what they clearly need to do. But I've also been the PhD guy, saying "yes sure" and agreeing to spend 30 minutes on a task that doesn't make much sense rather than spend 30 minutes arguing about it. I don't have the technical knowledge to speculate on how obvious it should have been that the test you proposed was right.