I'm afraid the waiter does not have the power to free it, unless the internal rules in the restaurant, say: "Ah, if a guest found flies in the dish, you can give him a free meal." Oh, which restaurant can stand it. Or practical it, the waiter first apologized, after all, it must be the restaurant is wrong, hygiene is not good enough, and then ask the foreman, I think the least this dish should be redone, and free of charge, this meal is unlikely to be completely free of charge, the customer should not be catty.