I think your grandmother may not be seeing the whole picture.
Right off the bat, the officer may have phoned the order in and is simply picking up. Perhaps this is a thing your grandmother is not familiar with.
Or good chance what happened is that the officer was squeezed for time, either because of mismanagement at his duty station or an unusual security incident took precedence over his lunch break. And that may be partly his conscious choice to place public service first. That is his chosen profession, after all.
So now, he's on a shortened lunch break or worse, trying to squeeze a late lunch into a 15 minute smoke break. From his POV, no knowing citizen would spite him 30 seconds in line if they knew of his completely lost lunch hour, and would feel bad for the shambles of a lunch break he is actually getting. But of course he doesn't have time to explain all that.
Conversely, your grandmother has a narrative of her own: that of authority figures abusing their power. The facts at hand fit like a glove into that narrative; she imagines the 5 minutes he should've spent in line will now be spent goofing off or whatever authority figures do when they're not humiliating citizens needlessly. She may not be correct. She may not care. This version of the truth lets her reinforce her preconceived notions, and there's a sense of security in that. And there's not a word the officer could've said to her that would've changed her narrative, and he knows that from his experience working around people.
Keep in mind, too, that the restaurant is choosing to serve him out of line. Can't blame that on authority; the officer doesn't have much power to harass the restaurant staff; they're supposed to be there and they probably control the security cameras. Now if someone came into my establishment and took cuts in front of customers without my permission, they wouldn't get served. Conclusion: The restaurant endorses expediting service to airport staff, and for good reasons to which your grandmother is not privy.
This seems to have completely escaped your grandmother's logic; she had every right to say "Harumph, I won't do business with you, complicit restauranteur, I'll go across the aisle to someone else!" At the least she could've asked for the manager and asked "Excuse me, why did you serve him out of line?" and listen patiently and without prejudice for a reasonable answer. She might've gotten one.
Honestly the restauranteur may be well aware of the situation and time demands on airport staff, and made that policy as a concession to their reality. Consider too that staff are regular customers in a way none but the busiest business traveler will ever be. At the end of the day, the restaurant belongs to the owner and he must do what's right for the restaurant. The customer is always right, but two customers at odds can't both be right.