Rover has a same-day cancellation policy for daycare. If the owner did not utilize that feature, that is on them. They should have read the terms before being a no-show. That's not fair to you, as you could have turned down another potential client for them.
I would also try to contact owners the night before and morning of their bookings. I always give a reminder to my clients about their bookings. Communication will help prevent situations like this.
If they try to leave a bad review, contact Rover support and share what happened. You can't do much else on your end, and having an uninsured stay would be too risky for both parties. Let them know they should contact Rover and let them handle it with them. What you can say is:
“Hi...,
I’m an individual contractor who works under Rover to ensure your pet’s safety during stays. Therefore, I have to follow Rover policies. I can not offer another day, as it would be under the table and uninsured by Rover. All daycare visits have a one-day cancellation policy, and if you utilize it, it gives a refund. Don't hesitate to get in touch with Rover about what happened, and they will help you through this process. There is nothing I can do on my end. Rover’s number:...”
What time was the dog supposed to be dropped off with you? I don't understand why you waited until 3.30pm to contact the owner.