Should he be charitable towards his brother and pray with him at a time when prayer is disallowed?
There is nothing wrong with repeating a prayer in congregation at a time when prayer is disallowed, as an act of charity towards one who missed the prayer in congregation and who cannot find anyone to pray with him, because this is not a naafil prayer that is done for no reason, rather there is a reason for it, and if there is a reason for a prayer, it is permissible to do it at a time when prayer is disallowed, according to the more correct of the two scholarly opinions.
The fact that it is prescribed to repeat the congregational prayer in this case is indicated by the report that was narrated by Ahmad (11631), Abu Dawood (574) and al-Tirmidhi (220) from Abu Sa’eed (may Allaah be pleased with him), that a man came when the Prophet (peace and blessings of Allaah be upon him) had prayed, and he said: “Won’t some man be charitable towards this man and pray with him?”
The version narrated by Abu Dawood says: The Messenger of Allaah (peace and blessings of Allaah be upon him) saw a man praying by himself and said: “Won’t some man be charitable towards this man and pray with him?” Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about the words of the Prophet (peace and blessings of Allaah be upon him), “There is no prayer after Fajr until the sun has risen high, and there is no prayer after ‘Asr until the sun has set” – is this general in meaning or not?
He replied: It is not general in meaning, rather there are exceptions.
Repeating a congregational prayer, such as if a person prays Fajr in his local mosque, then he comes to another mosque and finds them praying Fajr – he may join them and there is no sin or prohibition on him. The evidence for that is that the Prophet (peace and blessings of Allaah be upon him) prayed Fajr one day in Mina, and when he finished he saw two men who had not prayed with him. He asked them: “Why didn’t you pray?” They said: We already prayed in our camp. He said: “If you have prayed in your camp then you come to the mosque where there is a congregation, then pray with them.” And this was after Fajr prayer… End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (14/344).
And Allaah knows best.