The driver in the original accident is not responsible for paying for any damage inflicted on the hire car. He has no liability for that at all.
You were asked to check the vehicle and if it was not present or noted either when it came off the previous hire or when you received it the implication is that you caused it and the claim is against you.
If you are certain you did not cause it then -- if they pick it up when you take it back -- your best hope is that it was noted at the previous hand-back and you need the paperwork or you will be charged for it.
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