@Tami229
The "long term stay" cancellation policy is automatically applied on reservations of 28 nights and more, overruling the default set cancellation policy. It can not be manually applied to shorter stays. Changing to 25 nights means: no monthly discount, converting the reservation to the default set cancellation policy and also:a higher Airbnb Guest Service Fee (unless the host is on the host-only-fee structure)
Yoo can provide her a special price bij amending the price in the change form (if you want), but you can not change the cancellation policy other then what is in your settings now (flexible,moderate or strict) nor can you reduce her Airbnb Guests service fee to the lower "long stay" level.
I fear a 25 night booking will indeed cost her (much) more then the 28 nights booking she already has.
If the "long term stay" cancellation policy is her major concern, you can also make other agreement about a (partial) refund. in case of cancellation or shortening the stay..