How long does a customer have before they can report a defect from an item they received?

A customer ordered an item from us on November 1st, 2024 and then they received it on November 5th, 2024. They notified us almost 67 days later that the item had a yellow stain on the shirt. Due to Amazon's extended holiday return window, the order was eligible for a return. My question is I feel as if they have used the product and the product now has a stain on the shirt. This stain was never there before. We have sold numerous of this sku and have never heard of a complaint like this before. They are asking for a full refund. Can anyone advise on what the appropriate action is that we should take? Any advice is greatly appreciated.