How can one help a friend who has joined the group of Nasser Mohammed Al-Yamani (the Awaited Mahdi) and bring him back to the right path?

Sound hadiths indicate the emergence of the Mahdi at the end of time. There are signs for him that do not apply to the man in question, including: his physical description does not match what was reported, his emergence is from the East while the claimant is from Yemen, and his calling people to pledge allegiance to him while the Mahdi does not call for himself and does not know that he is the awaited Mahdi except when Allah wills his manifestation. He will refuse the pledge of allegiance until he is compelled to accept it. Sufyan al-Thawri's saying, "If the Mahdi passes by you while you are in your house, do not go out to him until people gather around him," is the optimal way to deal with claims of Mahdism.