Can anyone call a Muslim a Kafir if they witness them doing something that contradicts Islam?

Q 3: Is it permissible for me to say to a person who commits (Part No. 1; Page No. 426) an act contradictory to faith, O disbeliever?

A: Anyone who invalidates his faith and Islam and decisive evidence is established against him but insists on his disbelief is considered a disbeliever. His case must be submitted to the legal court to issue a verdict.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.