What is the ruling to say to someone he or she is a Munafiq?
Islam is a complete religion and teaches us manners in all aspects of life.
As Muslims, we are not allowed to swear at/slander /backbite any human being; be they Muslim or non-Muslim.
Therefore, calling a Muslim ‘Munaafiq’ would be a very grave sin.
The Prophet Sallalahu Alayhi Wasallam has said
عن أبي هريرة رضي الله عنه: أن رسول الله صلي الله عليه و سلم قال: إذا قال الرجل لأخيه يا كافر فقد باء به أحدهما (رواه البخاري)
(Translation) Hazrat Abu Hurairah رَضِيَ اللهُ عَنْه has narrated that the Prophet صلي الله عليه و سلم said, “When a person says to his brothers ‘O Kaafir (disbeliever)’ then verily one of them has returned with it”
The meaning of this hadeeth is that if the person, who was called a Kaafir, is in reality a Kaafir, then so be it, but if the person is not a Kaafir then the label of Kurf will return on the person who said it.