BlueBelle
You're describing me at the same age, very bright, lots of friends, but absolutely no self confidence at all.
Please, please don't even consider a Dr, Psychologist, or Therapist, she doesn't have any mental health issues, she's just a shy kid!
The thing that cured me was when my mum took me to see an old lady who she'd worked with many years before. Miss Lewis was a very small, frail, quietly spoken woman, and she didn't try to force me into joining in conversation, but told me that I could wander off and have a look around her house if I wanted.
I loved it, she was very old fashioned, her house was 'trapped in time' and being allowed to look around by myself was great!
Then, slowly but surely, mum started asking me to "pop around to Miss Lewis, she needs a bit of shopping done". Gradually, I started helping her out more and more, not realising at all that my confidence was growing all the time.
It took a few years, but now I'm a very confident person (have occasional shy moments, but hide them really well) and am nothing at all like that shy teenager,
She just needs to find the one thing which she enjoys so much she'll fight her shyness to enable her to do it.
A frind of mine was worried sick about her daughter's shyness, but encouraged her to volunteer at her local hospital, on the children's ward, she loved it, and never looked back. She's a well known paediatrician now, and absolutely loves her work, so it's just a case of 'finding the one thing'!
I wish her loads of luck for the future, she's young, gorgeous, no aches and pains, lots of energy, she needs to realise how lucky she is and enjoy her youth!