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. {C}
What this man did is labeled as lying, which is a sin; however, he cannot be declared a disbeliever merely because of lying and claiming that a given disbeliever is a Muslim. Verily, he whose Islam is established with certainty cannot be declared a disbeliever except with like certainty. For further benefit, please refer to fatwa 8106.
It is incumbent on him to repent to Allaah from the sin of lying; the conditions of sincere repentance were detailed in fatwa 86527. An aspect of perfection of the sincere repentance from lying is to seek pardon from the person he lied to, unless it leads to a graver harm; please refer to fataawa 81748 and 82148.
It should be noted that the Muslim should not let his tongue indulge and say whatever it wants to say. Verily, the evils of the tongue can lead the person to destruction. Allaah, The Exalted, says (what means): {Man does not utter any word except that with him is an observer prepared (to record).} [Quran 50:18]
Abu Hurayrah narrated that the Prophet
said, "A man would utter a word that is displeasing to Allaah without considering or thinking of its gravity, and because of that he will be thrown into Hellfire." [Al-Bukhari]
Allaah knows best.