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 is His slave and Messenger.
A Muslim does not become a disbeliever and is not considered out of the fold of Islam if he makes a mistake by declaring his fellow Muslim a disbeliever. The Prophet said: “Whenever one calls his Muslim brother ‘O Kafir (disbeliever),’ it would return (bounce back) to one of them.” [Al-Bukhari and Muslim] He also said: “Whenever one calls his Muslim brother ‘O Kafir (disbeliever),’ it would return to one of them. If he is not Kafir, it would return (bounce back) to the one who said it.” [Muslim] The Hadeeths aim to stress the gravity of Takfeer and labelling Muslims as disbelievers.
However, they do not mean that whoever labels a Muslim as a disbeliever has committed an act of major Kufr (disbelief) that takes the doer out of the fold of Islam if the accused person is not a disbeliever.
Allah knows best.