They would have to prove that they told her explicitly that, while she could drop the car off, any damage or loss was her problem. Unless they specifically made that point, the fact that they have a drop off service would imply that her responsibility ended when she dropped the keys in the slot, and any decent lawyer could make that case.
When the dollars get bigger, there is more responsibility to be explicit, and there is also the legal standard in Canada that someone in the business is assumed to be more knowledgable, and therefore has a higher standard of behavior.
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