If she cancels her booking, and it was made more than 48 hours ago, and rebooks the same dates, she'll get charged the service fees twice. You could advise her to contact Airbnb and explain the situation. It's up to them whether they will let her apply the coupon after the fact.
Whatever you do, if she cancels the reservation and you get a message asking you to approve the cancellation, DO NOT. Decline it. When hosts get a notice like that and accept it, it goes down as a host cancellation, for which you will be punished.