The Mufti is right. A Muslim calling another Muslim a unbeliever or the condemnation of a Muslim by another Muslim as a kafir is strictly prohibited in the Quran, the Hadith, and the writings of many eminent Muslim authorities.
According to the Quran, if a person says assalamu alaikum to us to indicate that he is a Muslim, we cannot say to him "you are not a believer.'' (4:94)
The second thing we learn from this verse is that if, from among a non-Muslim people, a person addresses us by assalamu alaikum, that is sufficient proof that he is a Muslim. When such incidents took place during the Holy Prophet's life-time, sometimes it was suspected by some Muslims that such a person was not sincere. But the Holy Prophet would say to them: ``Did you tear open his heart to see what was in it?'' http://www.islamopediaonline.org/fatwa/prohibition-takfir-muslim-calling-another-muslim-unbeliever
Determining whether someone is a kaafir or a true believer is not up to us, rather it is up to God, may He be exalted, and His Messenger (peace and blessings of Allaah be upon him). Actually, Most ISIS extremists call other Muslims "apostates/kafir/unbeliever".
