It is not permissible to pray behind one who claims to have knowledge of the unseen by reading coffee cups or palms or anything else, because claiming to have knowledge of the unseen is kufr.
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: Is it permissible to pray behind an imam who is a charlatan and a liar, knowing that some of them recite Qur’aan well?
If the imam is a charlatan who claims to have knowledge of the unseen or who does false and evil things, then it is not permissible to appoint him as an imam or to pray behind him, because the one who claims to have knowledge of the unseen is a kaafir – we ask Allaah to keep us safe and sound. Allaah says (interpretation of the meaning):
“Say: None in the heavens and the earth knows the Ghayb (Unseen) except Allaah, nor can they perceive when they shall be resurrected”
The same applies to the one who practices witchcraft – he comes under the same ruling as the kuffaar, because Allaah says (interpretation of the meaning):
“They followed what the Shayaateen (devils) gave out (falsely of the magic) in the lifetime of Sulaymaan (Solomon). Sulaymaan did not disbelieve, but the Shayaateen (devils) disbelieved, teaching men magic and such things that came down at Babylon to the two angels, Haaroot and Maaroot, but neither of these two (angels) taught anyone (such things) till they had said, ‘We are for trial, so disbelieve not (by learning this magic from us).’”
But if he commits some sins but does not do any of the actions that constitute kufr such as witchcraft or claiming to have knowledge of the unseen, but he does commit some sins, then prayers offered behind him are valid, but it is better to look for another imam who is of good character and is righteous, so as to be on the safe side with regard to your religious commitment and so as to avoid going against those scholars who say that it is not permissible to pray behind him. Sinners should not be taken as imams but when they are imams, prayers offered behind them are valid, because the people may be tested with them and it may be necessary to pray behind them. But as for the one who calls on someone other than Allaah or seeks the help of the dead and asks them for support, no one should pray behind him, because by doing those things he is a kaafir, as these are the actions of the mushrikeen whom the Prophet (peace and blessings of Allaah be upon him) fought in Makkah and elsewhere. We ask Allaah to set the affairs of the Muslims straight and to enable them to understand their religion properly, and appoint the best of them in charge of their affairs, for He is All-hearing, Ever Near.
End quote from Majmoo’ Fataawa Ibn Baaz (9/278).
The person asked about here should be advised and the truth should be explained to him, and the regular imam should be informed about him, so that he can prevent him leading the prayers in his absence.
And Allaah knows best.