If it is already esablished(running for a long time) and you tell us what fish she already has and what the size of the tank is then we could recemend a fish.Tell us as much about the tank as you know.
If you are getting her the tank as well than you could buy her a hardy fish to cycle the tank that same day.
Just remeber that teh fish can only last in that bag like a hour so are you going to buy the fish on christmas eve and give it to her early?
You cant buy the fish on sunday night and give it to her on monday morning the fish would be dead for sure.