Speaking for the US, when someone is accused of a crime, when they are on trial, they are considered innocent until proven guilty. I would not be running around telling people “Mr Jones IS a murderer” until Mr Jones has been found guilty beyond a reasonable doubt by the preponderance of the evidence.
On the other hand, if you witness Mr Jones killing someone, even if Mr Jones is Fr Jones or even Pope Jones, you are bound by charity to call the police.
You asked for proof, thing is, it is impossible to prove a negative, so we cannot prove the Church does not teach that priests are impeccable and beyond reproach. The duty is on your friend making the claim to provide evidence.
We are bound to speak the truth and exercise prudence in the Catechism:
2475 Christ’s disciples have "put on the new man, created after the likeness of God in true righteousness and holiness."274 By “putting away falsehood,” they are to "put away all malice and all guile and insincerity and envy and all slander."275
[2476] False witness and perjury . When it is made publicly, a statement contrary to the truth takes on a particular gravity. In court it becomes false witness.276 When it is under oath, it is perjury. Acts such as these contribute to condemnation of the innocent, exoneration of the guilty, or the increased punishment of the accused.277 They gravely compromise the exercise of justice and the fairness of judicial decisions.
2477 Respect for the reputation of persons forbids every attitude and word likely to cause them unjust injury.278 He becomes guilty:
- of rash judgment who, even tacitly, assumes as true, without sufficient foundation, the moral fault of a neighbor;
- of detraction who, without objectively valid reason, discloses another’s faults and failings to persons who did not know them;279
- of calumny who, by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them.
2478 To avoid rash judgment, everyone should be careful to interpret insofar as possible his neighbor’s thoughts, words, and deeds in a favorable way:
Every good Christian ought to be more ready to give a favorable interpretation to another’s statement than to condemn it. But if he cannot do so, let him ask how the other understands it. And if the latter understands it badly, let the former correct him with love. If that does not suffice, let the Christian try all suitable ways to bring the other to a correct interpretation so that he may be saved.280
[2479] Detraction and calumny destroy the
reputation and honor of one’s neighbor . Honor is the social witness given to human dignity, and everyone enjoys a natural right to the honor of his name and reputation and to respect. Thus, detraction and calumny offend against the virtues of justice and charity.
