Matthew 18:16-17

16But maybe he does not want to listen to you. Then take one or two other people with you to speak to him. They will then know what wrong things your friend has done. Remember what the Bible says: “There must be two or three people to say certainly that another person has done something wrong. Two or three people must agree what has happened.”

17If your friend still will not agree that he has done something wrong, then go and tell the church leaders. If he does not agree with the leaders, stop being his friend. He is like somebody who refuses to obey God.