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@Yannis21
You are obviously working with a strict cancellation policy.
If Your guest cancells now, today, Your guest will have to pay a 50% cancellation fee.
If Your guest does not cancel until the day of check-in and then doesn't show up, the guest will be able to leave a review, but he/she will have to pay 100% of the rental price. Because if a guest doesn't cancel 14 days prior to check-in under strict cancellation terms, no money will be refunded at all.
So yes, in case of no show Your guest can leave a review, but that's an expensive review.
If I was in Your shoes, I would just reject the alteration request. As Your guest didn't send any message with the alteration request, I would also just write nothing.