Of course it is. Mostly because this guy works his butt off, I'd guess that he was likely online at 6am, and will be online working while he's sitting at the tire place as well. Also because we're all adults here, and life happens.
If I needed to track his hours, though, it gets more complicated. "What time did you start this morning? Will you be working at the tire place? Let me check your schedule to see how many hours you've worked already this week. Were you planning to leave early today? Well...that's out the window." Or, I could just say "No" and be the bad guy. Or make him waste vacation time.
Like I've said, no winners.
On a side note the local university raised the pay of all post docs to above threshold levels so they didn't have to deal with the issue. I think most were near the threshold anyway. The average was a $2500 a year raise. I think most universities have done so.