All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Changing the Ha (ه) to Ha (ح) in the words of Allaah { اهدنا الصراط المستقيم } which means {Guide us to the straight path} [Quran 1:6] is considered a mistake that alters the meaning so, the prayer is not valid when reciting in the above manner.
The commentary of Ash-Shirbeeni, from the Shaafi‘i School, reads: "What is meant by altering the meaning is rendering the original meaning as meaningless or changing it to another meaning." [End of quote]
If this is the case, then the situation of the Imaam is one of the following two:
1. He is unable to correct the mistake because he is a non-Arab, or there is no one who can teach him. In this case, his prayer for himself (alone) is valid, and the jurists stated that the prayer of a person who recites Al-Faatihah while altering the meaning is valid if he is unable to learn correct recitation due to the absence of a teacher or due to time constraint, while he is willing to learn, as stated in Al-Fawaakih Ad-Dawaani by An-Nafraawi from the Maaliki School.
However, the jurists differed in opinion about the validity of the prayer of a person who is led by such a person, while he himself recites the Al-Faatihah correctly. Some of them are of the view that his prayer is not valid while others are of the view that it is valid but it is disliked for him to pray behind such a person. The Fiqh Encyclopedia reads: "The Shaafi‘i School, in their new opinion, and most scholars of the Hanbali School are of the view that it is not valid to be led in prayer by a person who omits one letter from the letters of Al-Faatihah or replaces the letter with another letter; this is also the standard view of the Hanafi School.... These jurists are of the view that such a person who omits one letter or replaces it with another letter, if he is able to correct his tongue (i.e. pronounce in a correct way) and he does not correct himself and correct the mistake, then his prayer for himself is invalid and it is not valid to pray behind him. In case he is unable to correct himself, like if his tongue does not permit him to do so, or due to time constraint, and he was not able to correct before that, then his prayer for himself is valid. If a person who is like him is led by him in prayer, then him praying behind him is valid because he is like him, so his prayer is valid.
"The Maaliki School and some Hanafi scholars, as well as Abu Thawr, ‘Ataa’, and Qataadah are of the view that it is valid to be led by a person who omits one letter or replaces it with another letter; this is the view chosen by Al-Muzani, but he conditioned the validity of being led by such a person on that his tongue does not allow him to pronounce it in a correct manner, or that he is able to correct but there is not enough time for him to learn; otherwise, being led by him is not valid." [End of quote]
2. If he is able to correct the error but he does not do so, the prayer of such a person is not valid as we have already mentioned. An-Nawawi said: "If one cannot read properly Al-Faatihah in its entirety – whether or not one memorizes it, or memorizes the whole thing except one letter, or pronounces a doubled (germinate) consonant as singular due to the laxity of his tongue, or does something else of the sort; and whether that is because of muteness or something else – then such an Ummi person (who cannot recite Al-Faatihah properly) and a person who has problems pronouncing some letters correctly, if he is able to learn, his prayer for himself is invalid and it is not permissible to be led in prayer by him without any difference of opinion among scholars on that. If he is not able because his tongue does not allow him or due to time constraint and he was not able to learn beforehand, then his prayer for himself is valid and if one like him is led in prayer by him, then the prayer of such a person is valid according to the agreement of the scholars." [End of quote]
With regard to what you mentioned about some worshippers in the Masjid saying that the prayer of a person led in prayer by a person who makes mistakes that alter the meaning is valid, this is not correct in an absolute sense; rather, if he purposely does so, his prayer along with that of the people praying behind him is invalid according to the agreement of the scholars.
Ad-Dusooqi said in his commentary on Ash-Sharh Al-Kabeer in the Maaliki Fiqh:
"To conclude, the person who makes mistakes that alter the meaning is one of the following:
1. If he made the mistake intentionally, his prayer along with the prayer of the people led by him is invalid according to the agreement of the scholars.
2. If he made the mistake out of forgetfulness, his prayer is valid according to the agreement of the scholars.
3. If he is unable by nature and he is unable to learn, then the ruling is the same (valid) because he cannot speak properly.
4. If he is ignorant and he is willing to learn, then there is a difference of opinion on him, whether or not he is able to learn, and whether or not he can be led in prayer by someone who does not make mistakes.
The most valid opinion about him is that the prayer of the people led by him is valid, and so is his prayer as agreed by Al-Lakhmi and Ibn Rushd, and there is no difference between a mistake that alters the meaning and a mistake that does not alter the meaning in all that we have mentioned."
We understood from the question that you are not sure about the mistake of the Imaam (whether or not it alters the meaning). If the matter is so, then you should not refrain from praying behind him. But if you are sure that he makes a mistake that alters the meaning, then as you see, there is a difference of opinion on the validity of the prayer behind him, so if you pray behind him and then you make up that prayer again in order to be on the safe side, then this is better in consideration of both views; otherwise, you should pray alone.
As regards the Friday prayer, you should seek to pray in another Masjid in your city, if any. If there is no other Masjid, you can pray the Friday prayer as Thuhr prayer according to the view of the scholars who consider that the prayer behind such an Imaam is invalid. But if you pray behind him, we would not place restrictions on something that some scholars said is permissible.
Allaah Knows best.