I want to raise a practical consideration for you. Make sure her parents are agreed to this plan - if you get her a pet and they don't want it, they may make it difficult for her to give it care. They may not be onboard with all the necessary water changes, for instance, because of water bill concerns, or won't want her to have a heater because of electricity bills. They may not get her medication if the fish is sick, or they'll believe what petshops say about their care.
I would definitely make sure you have their approval to give this gift.
If I were her, I'd be thrilled. :)