Read. She should read everything she can get her hands on. Bad, good, mediocre.
Watch movies, go to plays. Pay attention. Sometimes there's acting going on there.
I would also suggest that she get involved with her school's drama club, if there is one, and consider starting one if there isn't.
She should also get to know her friendly local community theatre groups. There she will meet other people with the same interest, and she'll be able to get advice about classes in everything from accents to dance - and most important of all, find out whether and to what extent she has a natural gift, and how hard she wants to work, and what she is willing to give up to develop it.