She is definitely asking for too much. Remind her that she is already getting a discount on her regular fees and give her two options. Pay the regular fee you would charge anyone else and pay half fees for her vacation time or pay the reduced friend fee and pay for the vacation time.

Alternately charge her 50% of the regular fee for the vacation days since she is not coming she doesn't qualify for the friend fee for these days.

Absolutely do not let her have the time at no pay. She is being paid her salary while she is on vacation and you deserve your salary for the space she has booked whether she chooses to use it or not.