Yes, corn does cross pollinate, but we were discussing the difference between an incidental natural cross between neighboring plants, and what actually constitutes a hybrid. Those are totally different things.
On a side note, isolation distances for corn is about 600 yards. That's about the maximum distance that viable corn pollen can travel. If the roundup tolerant volunteer corn was further out on the field than 600 yards from your brothers field, it's likely your neighbor did something dumb and blamed it on your brother's corn.
