One friend should therefore attempt to persuade another only of what is honorable, safe, palpable, and free. Nor should friends only be admonished, but they should also be chastised if there is need. In all things, we should preserve moderation, not because we ought to be sweetly indulgent of our friends’ faults, but so that our admonitions be without bitterness and our chastisements be without abuse.
However, in being indulgent and gentle, we should exercise a certain pleasing and honorable friendliness; but flattery, that nursemaid of vice, ought to be far removed from our gentleness: it is unworthy not only of a friend, but also of a free man. But if one is completely deaf to the truth, such that he simply cannot hear the truth from his friend, then we must despair of his salvation.
Therefore, as Ambrose says,
“If you discover any fault in a friend, correct him in private; if he will not listen to you, correct him again. For there is such a thing as good correction, and it is much better by far than a silent friendship. Even if your friend thinks you are doing him harm, correct him still. For a wounded friend is more bearable than shows of affection from flatterers. Therefore, correct your erring friend.”
However, above all else, you should avoid mental anger and bitterness in correcting a friend, lest you seem to be bent on venting your own spleen rather than truly correcting your friend.