What is the ruling of Islamic law on adopting a girl and attributing her to a father other than her biological one, while concealing her true origins, and what should be done by one who knows of this, especially when the girl reaches the age of marriage?

You must advise that girl by informing her of the truth about her lineage, because lineage has many rulings tied to it, such as marriage and inheritance. It is one of the most basic human rights to know one's true lineage. Prophetic hadiths have described concealing this matter as a crime. On the authority of Abu Hurayrah (may Allah be pleased with him), the Messenger of Allah (peace and blessings be upon him) said: "Any woman who introduces someone to a people who is not from them, then she is not from Allah in anything, and Allah will not admit her into His Paradise." This is proof of the prohibition of falsifying lineage, whether by a man or a woman. The wisdom behind this is to preserve human dignity and their right to know their original lineage.

Therefore, you must strive to clarify the matter to the girl, even if through private messages or email without revealing your identity. You should inform her of the truth about her lineage, along with advice to be patient, and that she should treat those who raised her with kindness and respect, but she must be aware of the Sharia rulings that apply due to her being a stranger to them. The girl's age and understanding must also be taken into account; if she is young and cannot comprehend the matter, it can be postponed until she reaches maturity.