The explanation: I gave my friend a steel ball but myself used a SuperMagnet sphere made of neodymium-iron-boron of the same diameter.
The rolling movement of the magnetic sphere inside the aluminum angle bracket creates a voltage of approximately 10 microvolts (Dynamo Principle).
The electrical current created by this voltage differential causes a magnetic field in the aluminum.
This magnetic field is out-of-phase with the magnet itself, i.e.
it moves against the magnetic sphere, acting like a brake.
For this reason the forward motion of the magnetic sphere is strongly reduced.