When someone starts asking for multiple exceptions to your policies, it's usually better to encourage them to cancel. Or, if it's low season, you need the booking, and you're pretty sure not to rent to someone else, make a clear exception to your policy.
I would let her know that "holding the dates" is harmful to you, and that your flexible cancellation policy is a courtesy to help guests deal with true last minute emergencies. It shouldn't be used as a backstop so they can book someplace else later.
I would absolutely check back periodically to ensure her trip is still on. This lets the guest know they are dealing with a physical person, not a booking service or a machine, and that her cancellation is not indifferent to you. She'll probably cancel, and you want that to happen sooner than later so you can find someone else.
Why not offer her a one night stay for almost the same price as the two night stay, or only a very small discount.
Good luck!