It is Shakespeare ( sort of, it's attributed to him and John Fletcher) but rarely done-the Gaoler's daughter from The Two Noble Kinsman. It is the right age and is funny and gentle-she is desperately in love with a guy who barely knows she's alive.
Also the girl from A Night Out by Pinter. Bit dark and bleak.
Try Margery Pinchwife from The Country Wife by Wycherley. Classical,funny and youthful.
The speech about cockroaches in Hush by Moira Buffini (can't remember the character title but the speech is very funny).
Joan La Pucelle (Joan of Arc) in Henry VI part 1-Shakespeare and totally brilliant and not done too often I imagine. Youthful and earnest and full of drama.
Please don't do Talking Heads. They were written for specific actors in mind, mostly and are very far from original material for audition. Also they are television scripts and this is a theatre audition.
Tell her good luck.