I think it matters a huge amount what the BK in question was doing. If they were just flipping beef patties and plant-based patties on the same cooktop with no separation or cleaning in between, then I think the plaintiff has a valid point. If you cook vegan food on a surface covered in beef fat, it's no longer vegan. If they're just using the same piece of stainless steel and it'd been cleaned beforehand then it's ridiculous.