This is tough. I've made accommodations for people that have unexpected things arise but this sounds like she just found something she likes better. How else did she end up with 2 reservations for the month of April?
Run the numbers, regular rate at shorter stay versus current reservation. It always helps me to write it down. If you decide to give her a break, you'll have to explain the reasoning, that she's not entitled to a refund under Airbnb long term stay policy, and that the proposed short stay isn't entitled to a discount.
I feel like the host ends up on the losing end most times, but have come to accept it, as it hasn't happen ed to me often.
You are smart to keep all communication on the platform.
You are within your rights to stick with the policy. Otherwise decide what you can reasonably live with and work on your explanation so she feels like she's getting something. Also get her to alter her reservation so the days are available again. Although you say they probably won't book, maybe some of them will.
You might need to ask Airbnb to make manual changes to the reservation, I'm not sure if the reservation will recalculate to remove the discount at this point.
This type of thing can be time consuming and draining.
Good luck.