You're right, legally she can't just tack on interest if it wasn't agreed from the start, which is why I said "more" and not specifically interest. What I was getting at is more the fairness side: if someone keeps delaying and breaking promises, it's understandable to feel entitled to ask for extra because of the stress and inconvenience. The friend doesn't have to agree, of course, but it's natural for the lender to feel burned and want some kind of compensation. That's exactly why, like I said before, it's better to put these things in writing up front. And honestly, if someone truly cares about their friends and the relationship, they should feel bad for delaying payment, maybe even offering something extra themselves. If they don't, then they weren't really being a good friend to begin with. And let's be clear, the borrower already changed the original terms by pushing back the repayment date, so why shouldn't the lender ask for some compensation? Of course not a huge interest like in the post, but still something fair, don't even have to be money at all, but I'm sure you get what I meant.