AA works best for me because of the relative ease of obtaining international upgrades-- this is very important for me because I fly on my own dime and therefore save a lot of money by upgrading on international flights.

I have friends who like CO OnePass and UA Mileage Plus a lot as well. The best program for your friend will depend on where she typically flies, what class her tickets will be booked in, and which airline she prefers.

If possible it is best to concentrate all flying on one airline and its partners to get status and rack up a lot of miles on one airline.