Plenty of guys run a lathe in short sleeves and nobody blinks. Long sleeve loose cuffs is the real issue, not sleeves in general. And half these shops don't even enforce a dress code, so the guy might not know better or might've been told it's fine. If the spindle grabs a cuff, yeah it's bad, but calling a grown man out over his shirt in front of the whole shop is how you get him to dig in and keep doing it. Maybe that's just how that shop runs and it ain't your business anyway.