Answer
(Fatwa: 362/366/N=1433)
According to the Ulama, your friend who calls himself Siddiq Deendar attributing towards the person i.e. Siddiq Hussain alias Siddiq Deendar alias Srichan Basu Yashwar, founder of Deendar Anjuman, he is kafir, apostate and out of folk of Islam due to his various kurfiya and shirkiya beliefs. And those who consider him leader and agree with his beliefs and thoughts they too are kafir, apostate and out of the folk of Islam. It is unlawful and haram to offer salah behind them, to marry them, to participate in their marriage or invite them in marriages etc. They are not follower of the belief of Ahl Sunnah wal Jamah at all. The beliefs of Ahl Sunnah wal Jamah are mentioned in the books of beliefs (like Aqidah al-Tahavi and Talimuddin etc). Hence, your friend is quite wrong to call himself Sunni. They are not Sunni at all, rather they are kafir and apostate. If you want to learn more about this group then read the booklet named ‘Bhed Ki Surat Me Bhediya, in Ahsanul Fatawa’ Vol. 1 p. 215-282). Also you can study the book “Sri Chin Basu Yashwar, Apni Tahrirat O Nazriyat Ke Ayine Me” written by Maulana Muhammad Abdul Qawi Sahib, published by Majlis Ilmiya, Andhra Pradesh, Akbarabad Malikpet, Hyderabad, AP, phone no. 040-24540177).
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband