Fly in a plane with that person. With enough fuel one day you guys shall reach at where you had started. This will prove that earth is round.
Response to comments:
All material objects apply the gravitational force on all other material objects. The earth is made up of atoms,molecules etc, all these constituent particles exert force on each other.
All the heavenly bodies in the universe are more or less spherical in shape. This is due to the fact that any material object of a given mass has least surface area if it is spherical in shape, lesser the surface area lesser the potential energy more the system becomes stable. So for a given mass the object tends to become spherical in shape.
What would happen if earth becomes cylindrical?
Then all the objects placed on the curved surface of the cylinder near the edge will fall towards the centre until the Earth becomes spherical because of the gravity. In fact every system tends to go from higher potential to lower potential state.
Why some meteorites are not of spherical shape?
This is only the case for meteorites of small size. As the size grows the meteorites becomes more an and spherical. The small sized physical objects does not have enough gravity to change their shape.