From what you wrote. It sounds like at the time the request about getting the keys the day before was made that you didn’t have anything on your schedule right before her sit.
Ideally, once you booked that sit in the other country you should have let her know for sure that picking up the keys the day before was not going to be an option
You said that she didn’t say anything when you mentioned this possibility so there was the assumption she would be okay with it. But if she didn’t say anything in response specifically, I would have made sure to get some sort of answer so we would all be on the same page.
Like others have said, there is definitely a way for you to be able to get the keys without her being there-- using some sort of lockbox is probably the best option.
If she feels uncomfortable with that or any other option for whatever reason, and you don’t think it’s possible for one of you to get there the day before she leaves, the only thing to do is cancel the sit.
I’m sure this is a frustrating situation to be in but it is a good learning opportunity about clear communication for future sits.
Hopefully something can be worked out and the sit will go ahead as planned for you.