What would you do if we hypothetically discovered that plants do indeed feel pain?
As it turns out, even if a person’s primary goal in life is to minimize the number of plants which are killed, he should still become a vegetarian.
This is because plants have to be fed to the livestock, and meat production is extremely inefficient. Only a small fraction of the food which is eaten by animals is converted into meat for human consumption. Therefore, by eating meat, not only do we cause animal death and suffering, we also end up killing many more plants than we would if we ate a vegetarian diet.
Quote from Animal Rights and Vegetarian Ethics by Eugene Khutoryansky
I think that intolerant carnivores love to to harrass vegans/vegetarians with this question.
He might just want to know how a good vegan would react…