Bet Din would feed him bread and water for a while, to make his Intestines and stomache shrink and tighten. THen they would feed him the barley, which will kill him since he wont be able to handle it.
I asked the same question, just because we believe the torah is emet, so the torahs justice system also is emet, we want to know that this man would be killed in our world of emet!