Can a refund policy be modified after the fact?

With a strict policy, she can get 50% back from Rover (as they gave her) "If canceled later than 12:00pm one week before the stay begins, a 50% refund is available for the first seven canceled days of the stay and a 100% refund is available for any additional days."

If she was hesitant about leaving her pup with you, a better plan for all involved would have been for her to book a shorter service, like a single day or night, to try out the arrangement.

Certainly, you are not required to give her anything more than what she got. You may opt to tell her that the reason for the strict cancellation policy is that sitters plan for their dog guests and limited spaces on calendars. I saw you're accepting 6 at a time, so if that was the case then, that may or may not be a reason worth mentioning. If you want to give her something, you could contact Rover support and ask if they can refund some additional amount (which likely would be challenging since rover already compensated you with the cancellation fee amount for that booking) or a more creative way would be to offer her a discount towards future service(s), which you can make as a Cost Adjustment. Maybe instead of boarding, her dog would enjoy a bunch of walks or drop in visits or even house sitting at her own home, which could convert her into another client for you.