@Gina1746 if you are hosting guests you should always have someone available locally in case of problems and a way of accessing spare keys.
Under Airbnb's T&C no you don't need to offer the guest a refund. Yes it was the guest's fault for losing the keys but in your situation I would offer a partial refund.
It's not the guests fault you didn't have a local co-host to help while you were away or spare keys that they could access via your cleaner or a lock box or similar.
I would offer around 20% .