If by flat surface you mean a level surface, can see how it might. A level surface being a sphere with a constant radius and center congruent with the center of the earth. The drop of a level surface is, as I recall, about .6 ft per mile (have to find my surveying book to be sure). If the velocity of your bullet was such that it would drop less than .6 ft in its first mile of flight, then it would actually rise in reference to level. Does your friend care if the bullet is fired at the north pole or the equator? Would the balance of gravity and centrifugal force (yes, I know it is a myth) at the equator be such that the bullet would rise above its initial level? Anyway, I'd be interested in hearing your friend's argument, as it contradicts elementary physics texts of the 1950's

Jim