She has repaid him at a higher interest rate than any bank offers.

Borrowing money from your girlfriend, parent, or in-laws will likely hurt your relationship with them. It doesn’t matter whether you pay the money back on time or not. The relationship changes from girlfriend-boyfriend to lender-borrower, and that is a totally different dynamic. Every date you go on, and every purchase one of you makes for both of you, will feel different because the fact that one of you owes money to the other person is constantly in the back of your mind.

That being said, if your family member or partner does borrow money from you and they pay you back in full with interest, you really need to let go of the resentment you have, if you have any, about having lent them the money. You might regret lending money to someone because it changes the relationship, but if you agreed to loan them money, you’ve got nobody to blame but yourself for how you’re feeling. Nobody can force you to loan them money, so if you choose to loan instead of gifting money, you have to be prepared not to get that money back.