Is it permissible, in Islamic law, to buy an expensive gift for a female friend as a thank you for her help, with the husband's approval, or are the children more entitled to this money?

Rewarding a gift is a Prophetic Sunnah and a religious requirement, based on the hadith: "Whoever does you a favor, reward him," as it fosters connection and love. If rewarding a gift conflicts with the needs of one's children, then the children take precedence, because those who are closer have a greater right to kindness, as Al-Nawawi said: "It is desirable to prioritize in beneficence the mother, then the father, then the children, then the grandfathers and grandmothers, then the brothers and sisters, then the rest of the unmarriageable kin among relatives by blood such as paternal uncles and aunts, and maternal uncles and aunts, prioritizing the closest then the next closest."