Backbiting

Salam, I have a question. Is overhearing someone speaking ill about another person without intervening haram? And what if that person they are speaking about isn’t muslim? Because I know that it’s haram to speak badly about other Muslims but I’m not sure if it applies to non Muslims as well. Jazakallah Khair.

wa'alaykumussalaam

Usually if I am in this situation, I will get up and leave. Usually it makes me feel uncomfortable, especially if I don't think the same way about the person they talk about.
But I found this info, now I know there is something else I should do in that situation. Maybe this info will help you as well inshaAllah

https://islamqa.info/en/answers/66699/listening-to-others-backbiting

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.
He replied:
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” [4:140]
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. End quote.
Majmoo’ Fatawa al-Shaykh Ibn Baz.
So strive to remind your colleague of the ruling on backbiting, and explain to him the punishment of the one who does that. Perhaps he will give up what he is doing of sin. Remind him that backbiting during Ramadhan is more sinful, and the basic principle is that you should avoid sitting with him if he persists in not responding to the command of Allah. But seeing as you work with him and you cannot leave the place, then avoid listening to him and stop paying any attention to what he says. You can use the means of threatening to report him to his superiors at work, or threaten to tell the people about whom he is talking. Perhaps if he does not fear Allah he will fear people and will stop backbiting about them, and you will be relieved of having to listen to his annoying talk.
And Allah knows best.

More info about backbiting against muslims and non muslims :

https://islamqa.info/en/answers/13611/backbiting-about-non-muslims

Thank you so much. Jazakallah Khair.