Contact the airline and they will append it to the reservation.
When you get to the airport wheelchairs are available at the doors. A Skycap will take her through Security all the way to the gate. When the aircraft is ready for boarding, the same person will come and take her down the jetway to the aircraft's door. Note this person should be given a tip.
Since the airline knows there is need for a wheelchair, one will be waiting with a Skycap at the destination airport, right at the aircraft's door. This person will take her as far as baggage claim, and even assist in getting baggage from the carousel. Note this person also should be tipped.