Why does the guest want to leave @Sally503 ? Has she asked for a refund?
If it is that she has just changed her mind, under the long term policy she booked under, she is not entitled to a refund - I find Airbnb Help Centre is a great,resource for reminding yourself about the basics of managing your Airbnb. Have a look there to remind yourself of details of Airbnb's long term cancellation policy.
She has already been with you 10 days and has to give a month's notice so from what you have said wouldn't be entitled to a refund.
For future reference if there is a problem because saying your listing wasn't as described, a key amenity wasn't available such as wifi then the guest could make a claim in some circumstances so it's probably best not to rely on the Airbnb income generated until post a guests stay.
Just let the guest know she needs to cancel via Airbnb and the cancellation will be processed under the long term policy she booked under.
If you wanted to you could tell her that if once she cancels you get a booking to replace hers, you could consider a refund for any days you are availble to book (this you would pro-rata under the discounted rate she booked under). She would still pay the Airbnb fees.
