Yes! Just like backbiting a Muslim is haram, backbiting a non-Muslim is also haram. However, if the non-Muslim person’s “Kufr” poses a risk of harm to Islam or Muslims, then backbiting them would not be considered haram.
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دلائل:
الدر المختار: (کتاب الحظر و الاباحۃ، 263/5، ط: دار الفكر)
وتحرم غیبتہ (الذمی) کالمسلم وظاہرہ انہ لاغیبۃ للحربی اھ۔
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.