Backbiting is a major sin. Allah, may He be glorified and exalted, says (interpretation of the meaning):
“O you who believe! Avoid much suspicion; indeed some suspicions are sins. 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 Allah. Verily, Allah is the One Who forgives and accepts repentance, Most Merciful”
It was narrated that Anas ibn Malik said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “When my Lord took me up into heaven, I passed by some people who had nails of copper with which they were scratching their faces and chests. I said: ‘Who are these, O Jibreel?’ He said: ‘These are the ones who used to eat the flesh of the people and impugn their honour.’” [Abu Dawood]
You have to denounce this colleague of yours and not approve of what he is doing.
It was narrated that Abu Sa’eed al-Khudri (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “Whoever among you sees an evil action, let him change it with his hand (by taking action); if he cannot, then with his tongue (by speaking out); and if he cannot, then with his heart (by hating it and feeling it is wrong), and that is the weakest of faith.”
Al-Nawawi (may Allah have mercy on him) said:
Note that the one who hears any backbiting about a Muslim should refute it and rebuke the one who says it. If he does not rebuke him in words he should rebuke him by his actions. If he cannot rebuke him by his actions or his words, then he should leave that gathering. If he hears any backbiting about his Shaykh (teacher) or anyone else who has a right over him or is one of the righteous and virtuous people, then he should pay even more attention to what we have said.
In the book of al-Tirmidhi we narrated from Abu’l-Darda (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Whoever protects the honour of his brother, Allah will protect his face from the Fire on the Day of Resurrection.” [Al-Tirmidhi]
Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him) was asked:
I am a young woman who hates backbiting and gossip. Sometimes I am with a group of people who talk about others and they start backbiting and gossiping. In my heart I hate and loathe this, but because I am very shy I cannot tell them not to do that, and there is no place I can go to get away from them. Allah knows that I wish that they would talk about something else. Is there any sin on me for sitting with them? What should I do? May Allah help you to do what is best for Islam and the Muslims.
There is sin on you for that unless you denounce the evil. If they accept that from you, then praise be to Allah, otherwise you have to leave them and not sit with them, because Allah, may He be exalted, says (interpretation of the meaning):
“And when you (Muhammad) see those who engage in a false conversation about Our Verses (of the Quran) by mocking at them, stay away from them till they turn to another topic. And if Shaytan (Satan) causes you to forget, then after the remembrance sit not you in the company of those people who are the Zalimoon (polytheists and wrongdoers)” [6:68]
“And it has already been revealed to you in the Book (this Quran) that when you hear the Verses of Allah being denied and mocked at, then sit not with them, until they engage in a talk other than that; (but if you stayed with them) certainly in that case you would be like them”
And the Prophet (peace and blessings of Allah be upon him) said: “Whoever among you sees an evil action, let him change it with his hand (by taking action); if he cannot, then with his tongue (by speaking out); and if he cannot, then with his heart (by hating it and feeling it is wrong), and that is the weakest of faith.” [Muslim].
And there are many other verses and ahadeeth (prophetic narrations) which say the same thing.
And Allah is the Source of strength.