What is the ruling on praying behind someone who moves two fingers during the Tashahhud, given that the movement of the second finger is consequent to the first?

The Sunnah in the Tashahhud is to point with one finger, which is the index finger (al-musabbihah). Pointing with two fingers is contrary to the Sunnah. If moving the index finger necessitates moving another finger, for instance, if they are stuck together, then there is no harm to the worshiper. However, if one intentionally acts contrary to the Sunnah while being able to restrict the pointing to the index finger, then he has erred and missed the reward of following the Sunnah. He should be advised and the Sunnah should be clarified to him. His prayer is valid because this action is minor and does not affect the validity of the prayer, and moving the finger is disliked but does not invalidate the prayer because it is a slight action.