Is it permissible to accept financial aid from a friend of the father to pay for a university installment, even though the family's financial situation is acceptable?

If the assistance is financial, to pay a tuition installment, there is no harm in accepting it as a gift, even if the person is not in need, based on the Prophet's (peace be upon him) acceptance of gifts, and the permissibility of voluntary charity for the wealthy.

However, if the assistance is obligatory charity, such as Zakat, it is not permissible for the wealthy.

As for assistance through influence or intercession to waive the installment, the matter reverts to the university's regulations. If they permit it, there is no harm. However, using influence to nullify a right is not permissible.