I can't think of any good reason, unless she either wants to go, has a problem, or is sexually active/close to it.
The only thing I can think is maybe she *wants* to go and went to mom instead of dad to ask? Possible she has some sort of period issue she is embarrased about and asked her mother not to discuss with anyone? But if she is comfortable about it and willing, I can't see the harm. Maybe she has questions about puberty/sex she prefers to ask a doctor?
Did her mother have related health issues around that age, or happen to be sexually active at a very young age? I can see those as reasons she might think it's a good age to bring her daughter.
I think I was around 15 the first time I went for an official gyn appointment with pap smear and everything, I was already sexually active then.
If she does go, her regular ped or family doc (I am assuming someone she knows and is familiar/comfortable with) should be able to do the appointment and exam for her.