What is the ruling of those who backbite, speak ill of, or make fun of Muslims in front of non-Muslims? Are they considered from those who fight against Muslims in alliance with non-Muslims?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

Backbiting is prohibited, and some scholars considered it as one of the grave major sins, so a Muslim should avoid it. For more benefit, please refer to fatwa 81748.

If a Muslim backbites his Muslim brother in front of non-Muslims, then this is a serious matter that may result in driving the non-Muslims away from the religion of Islam instead of encouraging them to embrace Islam and softening their hearts towards it. So this could be a reason of affliction to their hearts.

Indeed, among the supplications of the believers in the Quran is the following: {Our Lord, make us not a trial for the disbelievers and forgive us, our Lord. Indeed, it is You Who is the Exalted in Might, the Wise.} [Quran 60:5]

It means that they supplicated Allaah not to make them a reason of temptation for the disbelievers.

However, backbiting a Muslim does not necessarily imply being an enemy to the Muslim and an ally to the non-Muslim.

Allaah knows best.