Praise be to Allah.
The belief that Allah, may He be exalted, is everywhere is a false belief that is contrary to the view of the Muslims, which is belief that Allah rose above His Throne and is above His creation. The implications of this person’s view are abhorrent and repugnant in the extreme.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
This view has implications that are very, very false, because if you say that Allah is everywhere, this implies that He is in lavatories – Allah forbid – and in other places that filled with impurities and filth, and who would describe his Lord in such terms? It is not possible for any believer to describe his Lord in such terms.
End quote from Fataawa Noor ‘ala ad-Darb by Ibn ‘Uthaymeen
The one who has some knowledge and awareness of this matter must advise those who hold contrary beliefs, because this is a major issue of belief, especially if the one who holds contrary beliefs is the imam who leads the people in prayer. If he responds to the advice and adheres to the way of Ahl as-Sunnah, then praise be to Allah. But if he persists in this false belief, then there is a difference of scholarly opinion as to whether prayers offered behind him are valid. The view favoured by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) is that one may pray behind him, because he is not deemed to be a disbeliever just because he believes in this innovated view; usually no one falls into innovation except one who misinterprets things and is ignorant.
Shaykh al-Islam (may Allah have mercy on him) said: With regard to praying behind an innovator, this is a matter concerning which there is some dispute and is subject to further discussion. Hence if there is no other imam, as in the case of Jumu‘ah prayer which is held in only one place, or the Eid prayers or prayers during Hajj behind the imam who is leading the Hajj, these prayers may be offered behind any imam, whether he is righteous or otherwise, according to the consensus of Ahl as-Sunnah wa’l-Jamaa‘ah. Rather the ones who refrain from offering such prayers behind the imams are those who follow innovation, such as the Raafidis and their ilk who do not believe in praying Jumu‘ah or offering the prayers in congregation. If there is only one mosque in the village, then praying in congregation behind one who is an evildoer is better than praying on one’s own at home, lest that lead to him not praying in congregation at all.
But if it is possible to pray behind someone other than this innovator, then this is undoubtedly better. However, if he does pray behind (the innovator), then there is a difference of scholarly opinion concerning his prayer. The view of ash-Shaafa‘i and Abu Haneefah is that his prayer is valid. As for Maalik and Ahmad, in their madhhabs there is a considerable difference of opinion and discussion.
However this only has to do with innovation that is known to be contrary to the Qur’an and Sunnah, such as the innovations of the Raafidis and Jahamis and their ilk. But if the Sunnah is prevalent and well-known, and the one who goes against it is only one person, this is the matter concerning which there is a difference of opinion.
End quote from Majmoo‘ al-Fataawa (23/356)
Shaykh al-Islam (may Allah be pleased with him) gave the example of the worst type of innovations, which is the innovation of the Jahamis and Raafidis.
He (may Allah have mercy on him) also said:
One of the fundamental principles of Ahl as-Sunnah wa’l-Jamaa‘ah is that they pray Jumu‘ah and Eid prayers, and prayers in congregation, and they do not abandon Jumu‘ah and prayers in congregation as the followers of innovation, such as the Raafidis and others, do. If the imam appears to be good and has not openly committed any innovation or immoral actions, then one should pray Jumu‘ah and prayers in congregation behind him, according to the consensus of the four imams and other leading scholars of the Muslims. Not one of the leading scholars has said that it is not permissible to pray behind anyone except one whose thoughts and ideas are known. In fact, after the time of their Prophet, the Muslims continued to pray behind any Muslim who appeared to be good. But if a person prays behind one who is known to be an innovator or evildoer, when he has the option of praying behind someone else, then in that case the majority of scholars regard the prayer of the one who prayed behind that imam to be valid. This is the view of ash-Shaafa‘i and Abu Haneefah, and it is one of the two views in the madhhabs of Maalik and Ahmad. But if it is not possible to pray behind anyone except an innovator or evildoer, as in the case of Jumu‘ah prayer where the imam is an innovator or evildoer, and there is no other Jumu‘ah, then in this case one may pray behind the innovator or evildoer, according to the majority of Ahl as-Sunnah wa’l-Jamaa‘ah. This is the view of ash-Shaafa‘i, Abu Haneefah, Ahmad ibn Hanbal and other leading scholars of Ahl as-Sunnah, with no difference of opinion among them.
When there are a lot of innovations, some scholars suggested that one should pray behind someone who is known to him, but they said this by way of recommending, not saying that it is obligatory. It was narrated from Ahmad that he said this to someone who asked him. But Ahmad did not say that it is not valid to pray except behind one whose situation one knows.
So it is permissible to pray behind one who appears to be good, according to the consensus of the Muslim scholars.
The Sahaabah (may Allah be pleased with them) used to pray behind those who they knew to be evil doers, as ‘Abdullah ibn Mas‘ood and others among the Sahaabah used to pray behind al-Waleed ibn ‘Uqbah ibn Abi Mu‘eet, who used to drink wine and on one occasion had prayed Fajr with four rak‘ahs, and ‘Uthmaan ibn ‘Affaan had had him flogged for that.
‘Abdullah ibn ‘Umar and others among the Sahaabah used to pray behind al-Hajjaaj ibn Yoosuf and the Sahaabah and Taabi‘een used to pray behind Khalaf ibn Abi ‘Ubayd, although both of them deviated and promoted misguidance. [?]
End quote from Majmoo‘ al-Fataawa (3/280-281)
He (may Allah have mercy on him) also said:
Ahmad did not denounce individual Jahamis as disbelievers, or say of any of them that he was a Jahami and disbeliever, or say such things of anyone who agreed with the Jahamis in some of their innovations. Rather he prayed behind Jahamis who promoted their beliefs and subjected the people to trials and punished severely anyone who did not agree with their beliefs. Ahmad and his peers did not denounce them as disbelievers; rather he regarded them as believers and accepted their rulership, and he used to pray for them and regard it as acceptable to be led by them in prayer, perform Hajj under their leadership, and go out on military campaigns with them, and he regarded it as prohibited to rebel against them. But he denounced what they introduced of false beliefs that constitute major disbelief (kufr), even if they themselves were unaware that it constituted disbelief. He used to denounce it and strive to make them recant it as much as he could. So he combined obedience to Allah and His Messenger in promoting the Sunnah and the faith and in denouncing the innovations of the heretic Jahamis, with paying attention to the rights of the believers, both imams and commong folk, even if they were ignorant innovators and corrupt evildoers.
End quote from Majmoo‘ al-Fatawa (7/507)
To sum up:
This teacher is not a disbeliever just because he believes in this innovation, so prayers offered behind him are valid. But if you find any imam who is one of Ahl as-Sunnah wa’l-Jamaa‘ah, then it is preferable to pray behind him.
And Allah knows best.