Your friends says that an exterior angle can never be complementary to any of the interior angles in a triangle?

Two angles are complementary if their addition is equal to 90°.

As you may know, the exterior angle is equal to the addition of both opposite interior angles, so suppose that you have a triangle with two angles of 30° and one of 120° (because the addition of the 3 interior angles must be equal to 180°)

In this case, the exterior angle that is opposite of both interior angles of 30°, is equal to their sum, so this angle has 30° + 30° = 60°

Now, this angle of 60° would be complementary with bot 30° interior angles.

So given the right triangle, an exterior angle can be complementary to some interior angles.

reduce the friction between its moving parts

step-by-step explanation:

in fact, by reducing the friction between the moving parts of the machine, it is possible to reduce the energy wasted due to this friction; therefore, more input energy is converted into useful work, and this will improve the efficiency of the machine.