Do not magnify the faults of others
"Do not magnify the faults of others," say the wise. "But if there is something good about a man speak appreciatively about it." I myself, however, am bringing your faults into the open. But, to repeat, you must not bring to light the drawbacks of others but only their good qualities. See, even the crescent moon is cool and radiant. That is why Siva wears it in his matted hair, makes its beauty known to the world. The same Siva swallowed the terrible halahala poison concealing it from everyone, so says Dandin in one of his poems.
Pointing a finger at the faults of others or exaggerating them in speech and writing has become the practice today. The more learned a man is, the more eager he is to find fault with others. "Finding fault is indeed the work of a vidvan," it is said. "The word vidvan itself is said to mean a dosajna." But a dosajna is one who knows the faults of something or somebody, not one who reveals them to the world or exaggerates them. If you think a person has any drawbacks you must speak to him about them in a friendly manner [so that he may correct himself] but not constantly harp on them and expose them to the outside world.
We must be worthy enough to speak about the faults of others and we cannot take upon ourselves the role of an adviser when we need to correct ourselves. Advice given by us then would be counterproductive. If we tell a man what is wrong about him he might even feel boastful about it. When are we fit to advise others? When we are worthy enough and when we know that our word will have the desired effect.
http://www.kamakoti.org/hindudharma/part22/chap10.htm