Electrostatic force, which makes the ring to rotate, is in the direction of electric field and is parallel to ground.
We do not have any force that has component normal to ground,
hence rotating ring will not experience any normal reaction force and firction force.
Weight of the ring, gravitational force due to mass of ring acts normally.
But in this case it can be assumed mass of ring is negligible.
Hence electrostatic force is much greater than gravitational force.
Hence it can be concluded, friction force is zero