What is the ruling on using money that a mother borrowed from the bank with unlawful interest? And what should be done about it?

The impermissibility of a usurious loan is tied to theibility of the borrower, not to the specific use of the money or the loan amount itself. Therefore, it is not forbidden to benefit from the loan in permissible ways. The mother must repent by feeling remorse, seeking forgiveness, and resolving not to return to the sin. She is obliged to expedite repayment if that frees her from the prohibited interest and without causing harm or hardship.