I hope your friend has discussed this with her parents. If they are taking the position of "you can have it if you can pay for it" then she needs to put on paper all the costs involved with owning a horse.
Here in Maine, we have blueberry picking season. Mostly kids too young to get a job any place else will do this hot, back breaking job. There's also babysitting, but that will take a while to earn enough.
My concern would be how will an unexpected expense (like an injury and vet bill) be handled? There is so much more to owning a horse than the initial buy cost.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".