Is backbiting and disrespecting a Muslim in front of non Muslims kufr?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Backbiting is forbidden and it is one of the grave major sins according to the view of many scholars.
Allah Says (what means): {Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah; indeed, Allah is Accepting of repentance and Merciful.} [Quran 49:12]
Backbiting a Muslim in front of a Kaafir (non-Muslim), is a heinous act because it is rejoicing the non-Muslim over the Muslim.
However, this is not Kufr in any case.
It is obligatory on the Muslim to abandon backbiting in all cases, and repent from it whether it is in the presence of a Muslim or a non-Muslim.
According to many scholars, to fulfill repentance from backbiting, it is sufficient for one to ask Allah to forgive the one whom a person has backbitten and to mention him with goodness to the same extent that he has backbitten him.
For more benefit, please refer to Fatwa 317041.
Allah knows best.