This action is clearly haraam, because it involves attributing himself to someone other than his father and his tribe, and because it involves lying and deceit, and it may lead to other wrongful things that have to do with inheritance and who one’s mahrams are.
Al-Bukhaari (3508) and Muslim (61) narrated from Abu Dharr (may Allaah be pleased with him) that he heard the Prophet (peace and blessings of Allaah be upon him) say: “Any man who knowingly claims to belong to someone other than his father is guilty of kufr, and anyone who claims to belong to a people when he is not one of them, let him take his place in Hell.”
Al-Bukhaari (6767) and Muslim (63) narrated that Sa’d (may Allaah be pleased with him) said: I heard the Prophet (peace and blessings of Allaah be upon him) say: “Whoever claims to belong to someone who is not his father, knowing that he is not his father, Paradise will be forbidden to him.”
What your relative has to do is tear up this paper and attribute himself to his father and his real tribe, and repent to Allaah, may He be exalted, for what he has done. The Shaykh of the tribe must also repent to Allaah, may He be exalted, for lying and cooperating in deceit.
The Standing Committee for Issuing Fatwas was asked about what people often do in order to obtain a passport, which is testifying that So and so was born in Bahrain, whether they are certain of that or not. Is this a type of bearing false witness?
It is not permissible for a person to give testimony except with regard to that which he knows by seeing or hearing it, because Allaah says (interpretation of the meaning):
“except for those who bear witness to the truth knowingly”
“And follow not (O man, i.e., say not, or do not, or witness not) that of which you have no knowledge. Verily, the hearing, and the sight, and the heart of each of those ones will be questioned (by Allaah)”
It was narrated that Ibn ‘Abbaas (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) was asked about testimony, and he said: “Do you see the sun?” He said: Yes. He said: “Bear witness to something like that or else leave it alone.” Narrated by al-Khallaal.
Based on the above, it is not permissible for anyone to bear witness that So and so was born in Bahrain unless he knows that. Whoever bears witness that So and so was born in Bahrain when he knows that he is lying, this is false witness, and he is included in the warning that is proven in the Qur’aan and Sunnah. End quote.
Fataawa al-Lajnah al-Daa’imah (23/505).
If this has to do with lying about one’s place of birth, then how about if one changes one's lineage and tribe?
The fact that this man could not find work, or has to support a family, or his father knows about that, does not justify him committing this great evil.
Whoever fears Allaah, He will protect him and suffice him and provide him from (sources) he never could imagine.
We ask Allaah to set our affairs straight and those of all the Muslims.
And Allaah knows best.