Hello Keyvan and all,
This one would be nice to have a definitive answer for.
On the one hand, there are these which forbid comments even if true in a Baha'i context (unless to an Assembly or Auxiliary Board member):
"Even if what is said against another person be true, the mentioning of his faults to others still comes under the category of backbiting, and is forbidden."
(From a letter written on behalf of Shoghi Effendi to the National Spiritual Assembly of the British Isles, February 11, 1925)
"Abdu'l-Bahá does not permit adverse criticism of individuals by name in discussion among the friends, even if the one criticizing believes that he is doing so to protect the interests of the Cause. If the situation is of such gravity as to endanger the interests of the Faith, the complaint, as your National Spiritual Assembly has indicated, should be submitted to the Local Spiritual Assembly, or as you state to a representative of the institution of the Counsellors, for consideration and action. In such cases, of course, the name of the person or persons involved will have to be mentioned.
(From a letter written on behalf of the Universal House of Justice to an individual believer, September 23, 1975)
“There is a clear distinction between, on the one hand, the prohibition of backbiting, which would include adverse comments about individuals or institutions made to other individuals privately or publicly, and, on the other hand, the encouragement to unburden oneself of one's concerns to a Spiritual Assembly, local or National (or now, also, to confide in a Counsellor or Auxiliary Board member)."
(Criticism : A Letter on Behalf of the Universal House of Justice)
On the other hand, I believe the following may weigh on the side of permitting such well-intended warnings:
"Kindness cannot be shown the tyrant, the deceiver, or the thief, because, far from awakening them to the error of their ways, it maketh them to continue in their perversity as before. No matter how much kindliness ye may expend upon the liar, he will but lie the more, for he believeth you to be deceived, while ye understand him but too well, and only remain silent out of your extreme compassion."
(Selections from the Writings of 'Abdu'l-Baha, section 138)
"...When a difficulty is brought out into the daylight and freely discussed by a duly authorised and responsible group of people who are sincerely desirous of finding the best solution and are free from prejudice or personal motive, then there is a good chance of overcoming it, but discussion of the faults of others behind their backs by unauthorised people who have no authority to take action in the matter, is surely one of the most fertile causes--probably THE most fertile cause--of disunity, and the importance of putting an end to this practice should be impressed on all Bahá'ís."
(From a letter written on behalf of Shoghi Effendi to the National Spiritual Assembly of the British Isles, followed by an extract from the Guardian's postscript, February 11, 1925)