What would happen if an exclusively carnivorous animal were to eat plants only?

The carnivore would most likely die in that situation. Its guts cannot digest vegetation well because it does not have the microbes that do the digestion in its guts, so it is unable to obtain the nutrients its body needs. There are plenty of carnivores, such as bears and canines, that may eat a high percentage of vegetation if they have to, but they don't eat vegetation exclusively, unlike your hypothetical example. Some carnivores, such as the giant panda, have adapted over geological time to an almost exclusively vegetarian diet, and indeed practically all mammalian herbivores evolved from insectivorous and even carnivorous ancestors, so it can be done if given time to adapt. But an obligate carnivore probably cannot adapt to a vegetarian diet within a single lifetime inside a zoo..