All this has been said regarding secret sins. But where the sin is quite public so that the judge and everybody know it you can without any sin avoid him and let him go, because he has brought himself into disgrace, and you may also publicly testify concerning him. For when a matter is public in the light of day, there can be no slandering or false judging or testifying; as, when we now reprove the Pope with his doctrine, which is publicly set forth in books and proclaimed in all the world. For where the sin is public, the reproof also must be public, that every one may learn to guard against it.
Luther's Large Catechism, Eighth Commandment, #284, Concordia Triglotta.