Someone used my credit card and we agreed that she is responsible to pay the bill on time. She didn’t pay on time and we now have to pay interest on her balance, plus a late payment fee. Is it ribis if she pays it?
Generally the owner of the credit card pays the company and the lender pays him back. It would not be permitted for him to take back the payment of interest from the borrower. This is because the original payment of the borrower (the person who borrowed the card) is considered a loan from the owner of the card to its borrower, and it is therefore forbidden to take back a larger sum than was initially borrowed.
This is the ruling issued by Bris Yehuda (Ikkrei Dinim 14:25) and by Nesivos Shalom (174:4:57). Mishnas Ribis (17:7) goes further and prohibits the repayment even when the borrower pays directly to the credit card company.