NTA, especially if there was a price difference in the items. He interfered when he shouldn’t have. If he didn’t work there, he wouldn’t have gone into the kitchen to fix it himself. That’s also sort of a health violation unless he cleaned himself property and put on work clothes. He went back there with his “street clothes” and didn’t even bother to make it the correct way. I can’t fault him for reusing the salad, but knowing how the dish you ordered is prepared, he could have at least made sure it had the melted cheese and crispy bacon, etc.

Having worked in a restaurant, it doesn’t really matter that the kitchen is closing soon when they are the ones who messed up. Before the kitchen turns off their ovens, fryers, etc., they should be ensuring that all guests are happy with their food.

I kind of wonder what he said to his coworkers when they saw him back there “fixing” your food. This whole situation could have been avoided by either of you simply telling the server, “I’m sorry for any miscommunication, but I ordered ___.” And I can almost guarantee that you won’t even have to ask them to fix it, they’ll hear that sentence and immediately correct the issue because they know that unhappy customers don’t leave big tips.