I may be wrong but you should be able to use the law of cosines. I drew up a crappy diagram in MS paint...
Yfrog - anglebetween.jpg
Use the north pole, find the distance between the north pole and point A, the north pole and point B, and between A and B. You are trying to find the angle marked by the X, or angle A. I'll call the sides opposite of A, a, and B, b, and the north pole is point C, so the side opposite that is c.
a^2 = b^2 + c^2 - 2bc Cos(A)
so A (the angle X in the diagram) is = arccos ((a^2-b^2-c^2)(-2bc))
And A will give you the bearing from the North, and keep in mind arccos doesn't give you negative angles (only 0 degrees to 180) so you will have to determine (easily) if the person is going west or east. So if you get 90 degrees from this, or pi/2 radians, see if point B is to the east of A or west of A, and if its east of A then the person is going directly east, and vice versa.
