Geometrically speaking, a circle is defined as the set of points that are a certain distance from a center point. An area burst N is circular - it includes all squares that are N or fewer squares away from the origin square.
In 4e battle-grid geometry, all circles are also squares, but not all squares are circles. An area burst is fundamentally defined as being circular (in the equidistant sense). If you rotate the square, it ceases being a circle.
Encourage your player to look into feats like Enlarge Spell and War Wizardry, and items like Architect's Staff.
This time, McCoy squared the circle.