The questioner (...) from Ha'il inquires, "I have a friend who is of the habit of talking about people behind their back; I advised him to stop but in vain. Sometimes he would say such things with a good intention. My question is: Should I abandon him?"
A: Saying things about Muslims which they will dislike to hear is a grave Munkar (an act which is unacceptable or disapproved of by Islamic law and Muslims of sound intellect) and is included in prohibited Ghibah (backbiting). It is also one of the major sins, because Allah (Exalted be He) says, And spy not, neither backbite one another. Would one of you like to eat the flesh of his dead brother? You would hate it (so hate backbiting). And fear Allâh. Verily, Allâh is the One Who forgives and accepts repentance, Most Merciful.
Do you know what is Ghibah (backbiting)? They said, "Allah and His Messenger know best." He (peace be upon him) said, "Your mentioning your brother with something about him that he dislikes." It was said, "Tell me, if what I say about my brother is truly (found) in him? He (peace be upon him) said, "If what you say is truly in him, then you have backbitten him; and if it is not in him, then you have slandered him."
The Prophet (peace be upon him) saw on the night of Mi'raj (Ascension to Heaven) people who had copper fingernails with which they scratch their faces and chests. He asked Jibril (Gabriel), "Who are these?" He replied, "These are the ones who eat the flesh of people (backbite them) and impugn their honour." (Narrated by Ahmad and Abu Dawudwith a good Isnad [chain of narrators] on the authority of Anas)
Indeed, among the major sins is attacking the honor of a Muslim without right.
Therefore, you and other Muslims should avoid sitting with those who backbite Muslims and advise them to abandon this ill deed. The Prophet (peace be upon him) said, Anyone of you who sees an evil action, let them change it with their hand (by taking action); if they cannot, then with their tongue (by speaking out); and if they cannot, then with their heart (by hating it and feeling that it is wrong); and that is the weakest of Iman (faith). (Related by Muslim in his Sahih)If the person does not comply with the advice given and desist, then do not sit with such a person for this is a full expression of disapproving their act.