Praise be to Allaah.
This action is haraam. It is not permissible for a person to claim to belong to anyone other than his father, because this leads to further lies, and has an impact on matters of inheritance, who is considered to be a mahram, and all other issues that have to do with lineage. Thus the texts issue stern warnings to those who claim to belong to anyone other than their fathers. The Prophet (peace and blessings of Allaah be upon him) said: Whoever claims to belong to anyone other than his father on him will be the curse of Allaah and the angels and all the people.(Reported by Ibn Maajah, 2599; Saheeh al-Jaami, 6104). This action is also a major sin comprised of two major sins, lying in order to consume wealth unlawfully and claiming to belong to someone other than one's father.
This person has to go back to what is true, and give up the nationality he aquired improperly. He should go back to using his proper documents that prove his real lineage. This is obligatory upon him. I am astounded that anyone would do such a thing for worldly gains. Allaah says (interpretation of the meaning):
Whoever wishes for the quick-passing (transitory enjoyment of this world), We readily grant him what We will for whome We like. Then, afterwards, We have appointed for him Hell, he will burn therein disgraced and rejected, (- far away from Allaahs Mercy). [al-Isra 17:18]
The one who does this had to repent to Allaah, and Allaah has forgiven what is past [al-Maaidah 5:95 interpretation of the meaning]. With regard to whatever money he has taken as a result of having that nationality, Allaah says (interpretation of the meaning): (peace and blessings of Allaah be upon him) So whoever receives an admonition from his Lord and stops eating riba [usury] shall not be punished for the past [al-Baqarah 2:275]. This aayah is speaking about the one who consumes riba (interest or usury), so it applies even more in the case of lesser sins. And Allaah knows best.