How do you treat someone who is wrong?

Going around holding someone in thought as wrong, mentally working to correct their wrongness, is itself wrong practice, or mental malpractice. It manifests itself as hatred, and does the most profound damage to the one who is indulging in the malpractice. The one who "is wrong" may feel the hatred, but the hater is rotting his own soul, so to speak and will pay the price of spiritual emptiness, until he stops malpracticing. This is amplified when a group works "in unity" along these lines. 

In correct practice, in treating someone who is wrong, we start with numero uno, and turn to our Maker, divine Intelligence, first to declare "Perfect Principle and idea, perfect God and perfect man" (S&H) as our starting point. If we have enough love, we might rebuke the sin that we perceive in another, not to fix that one, but to break the mesmerism that would be binding him. (For example, a child might be whining and self-involved. A sharp rebuke can break the self-perpetuated unhappiness, and the child will be free and happy.) Beyond this, we mentally declare "the wrong one" to have the one Mind, and leave him to his Maker, Who knows far better than we how to handle the situation.

"Knowing that God is the one Mind, that He is all the Mind there is, you cannot be duped into attempting to deal with another mind." (Eustace)