Unless the diner is a Michelin star inspector or a food critic (and sometimes not even then), the chef in a very posh restaurant doesn't want your opinion or 'useful response' or 'shit sandwich' about the food.
Their opinion very much is that this is the way they intended the food to be and if you don't like it, well you know not to order it/come here, again.
If you liked it, you ate it all.
If you didn't eat it all then you probably didn't like it (they might come out and ask you what was wrong, they may actually want to know or they may want to see what heathen pigdog doesn't like their food so they can blacklist you, hunt you down... I've been to too many french restaurants with psychotic chefs)..or you were full.
If there had been a serious issue, you'd have said so fairly early on.
This isn't like a friend or partner honestly asking 'oh I had a crack at X dish, what did you think?' where the 'well the carrots were great, but the beef under seasoned, loved the parsnips' is useful and wanted.
So someone making 'constructive comments' about the food in a high end restaurant, particularly where this makes other diners in their party feel uncomfortable, is not behaving reasonably. They're being a twat, showing off, deluded in the misconception their opinion carries any weight or will even make it back to the chef (unless as a joke 'haha, the troglodyte on table 3 thinks the beef was a shade over and the consomme could have been clearer, he'd like to show you how he does it when he cooks it for his Mother In Law... NO CLAUDE PUT THE BONING KNIFE DOWN we only just got the blood out of the carpet from last time...')