An American group denying what is mentioned in the Qur'an

Q: An Imam of a Muslim group in America, whose name I prefer not to mention, reiterates some heretic claims. For example, they claim that Prophet `Isa (Jesus) was actually crucified; that Allah did not part the sea for Prophet Musa (Moses), that it is permissible for a Muslim woman to marry a Christian, and that Christians are better than Muslims in general. When the Imam of such group heard the Ayah of the stoning penalty, he described it as a barbaric penalty that should not be enforced. As for the Four Madh-habs (Hanafy, Maliky, Shafi`y, and Hanbaly schools of Jurisprudence), he claims, they are confined to Arabs in practice, thus he founded his own Madh-hab that suits the Americans, in addition to other false lies. As for himself, he claims he is the Christ and the Mahdy (a man named after the Prophet, who will appear as a sign of the Hour and rule according to Islamic law). However, later he abandoned his personal claims. A man from this group who worked as an Imam in his community twenty years ago came to our country and learned that such beliefs are false. Gradually, he learned the true Salafiyyah (the true creed of the righteous predecessors). He thus swore by Allah that he had never heard of the Salafiyyah except when he came to our country. Identifying the disbelief of the Imam mentioned above, we asked him to dissociate himself from that Imam and he acknowledged that the Imam is a Kafir (disbeliever) but he did not want to part with the group for the purpose of setting things right from the inside.Now, how should we deal with that man? Is it permissible for him to stay involved in this group under the pretext of inviting its people to the true Islam? Should he distance himself from that Imam? If he refuses to leave this group, should we abandon him?