Searching the website of sheikh al-Munajjjid, we found out that he does permit backbiting non-Muslims in absolute terms. The following is his fatwa regarding the issue:

Firstly, it is not a characteristic of a Muslim to speak rudely. The Prophet (peace and blessings be upon him) said: “The Muslim does not slander, curse, speak obscenely or speak rudely.” (Narrated by al-Tirmidhi). Whoever becomes habituated to do something, it becomes his habit. So the Muslim should keep away from all the ways that lead to evil, whether minor or major. Whoever drives his sheep near the enclosure of the king is bound to cross the line.

Secondly, if your question is referring to backbiting about a kafir by mentioning his physical defects, such as his having a long nose or a big mouth, etc., then do not do this, because this is making fun of the creation of Allah. If it means talking about his bad characteristics which he flaunts openly, such as zina and immorality, or drinking alcohol, and warning against him, there is nothing wrong with that. There follow some of the comments of the scholars on this topic:

Zakariya al-Ansaari said: “Backbiting about a kafir is haram if he is a dhimmi [a non-Muslim living under Islamic rule], because that puts them off from accepting the jizyah and it is going against the treaty of dhimmah (agreement between non-Muslim subjects and the Islamic state) and the words of the Prophet (peace and blessings be upon him). ‘Whoever makes a snide comment to a dhimmi has deserved Hell.’ (Narrated by Ibn Hibbaan in his Saheeh). It is permissible (to backbite about a kafir) if he is a harbi (one who is at war with the Muslims), because the Prophet (peace and blessings be upon him) used to command Hassan to lampoon the mushrikeen.” (Asna al-Mutaalib ma’a Haashiyatihi, vol. 3, p. 116)

Ahmad bin Hajar al-Haythami said in al-Zawaajir ‘an Iqtiraaf al-Kabaa’ir (vol. 2, p. 27):

“Al-Ghazali was asked about backbiting about a kafir. He said: with regard to a Muslim, it is forbidden for three reasons: causing offence; criticizing the creation of Allah, …; and wasting time in something that is of no benefit. The first is haram, the second is makrooh, and the third is not the best thing that one can do. With regard to the dhimmi, he is like the Muslim as far as not harming him is concerned, because the Lawgiver protects his honor, blood and property…. Ibn Hibbaan narrated in his Saheeh that the Prophet (peace and blessings be upon him) said, ‘Whoever makes a snide comment to a Jew or a Christian deserves Hell.’ The meaning of making a snide comment is to make someone hear something that will cause offence to him. There is no stronger evidence than this, i.e., it is haram. Al-Ghazali said: with regard to the harbi, the former is not haram, and the second and third are makrooh. With regard to one who commits bid’ah (innovation), if he becomes a kaafir thereby, then he is like a harbi, otherwise he is like a Muslim, but speaking of him with regard to his bid’ah is not makrooh. Ibn al-Mundhir said, concerning the words of the Prophet (peace and blessings be upon him), “It is your mentioning about your brother that which he dislikes,” – this indicates that in the case of one who is not your brother, such as a Jew or a Christian or a follower of any other religion, or one whose bid’ah has put him beyond the pale of Islam, there is no backbiting in his case.”