This is probably not the answer you'll like, but if you're really only talking about literally (only) hours of unpaid work, I'd suggest you just do it and chalk

This is probably not the answer you'll like, but if you're really only talking about literally (only) hours of unpaid work, I'd suggest you just do it and chalk it up to "customer service" and hope that you're commitment to helping them even when it's not your problem will get you a good reputation for future work. I've seen this sort of thing too, and while it's frustrating when my code is blamed for user error or other people not reading the API I give them, I feel like it's more trouble than it's worth to stress out about it. Just give it a low priority, and do it when you have some down time in the interest of keeping future potential customers happy. Granted, I'm not some pro freelancer, but doing this has let me cultivate many long term positive relationships, learn new skills on someone else's dime, and get new work with (potentially better) clients based on the recommendation.