Cases in Which Ghibah (Backbiting) is Permitted
Question

What are the situations in which it is permissible to talk about my Muslim brother in his absence and it will not be regarded as Ghibah (backbiting)?
Praise be to Allah.Ghibah (backbiting) means saying something about one’s Muslim brother that he dislikes, such as criticizing him and running him down and the like. But there are some situations mentioned by the scholars in which a Muslim may talk about his brother if that serves an interest. These situations include asking for one’s rights from a wrongdoer, so he may say to the judge or ruler, for example: So and so wronged me by doing such and such. 
Another case is seeking a fatwa (religious verdict), where the person may say to the mufti (scholar): So and so did such and such to me; does he have the right to do that or not? 
Another case is warning the Muslims against evil or dubious people, such as exposing those in whose character is something questionable with regard to narration (of hadeeth) and testimony.  
Another case is asking about a person in cases of marriage, business or neighbourhood. 
Another case is mentioning by name those who commit evil openly. 
Another case is identifying a person if there is no intention of belittling him, so that he is known by a nickname such as al-A’mash (bleary-eyed), al-A’raj (lame), al-Asamm (deaf) and so on.  
And Allah is the Source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote. 
Standing Committee for Academic Research and Issuing Fatwas.