That's a lot of 19s!
One possibility is for her to take the SATs now, as a 13 year old 8th grader. That might open up a few eyes.
With the parents, I would first try to get a look at the IQ test and see is she qualifies for YSP. (I know the numbers are a bit old, but this is a bad situation) If she does, then maybe you can sell them on 'free advice' for underachieving kids.
If you can get them to a Ruf lecture, that would be ideal.
If not, I would point out that she has been in the gifted program all through elementary school, and they don't let everyone in there, and the challenge is to turn potential into achievement.
Are there any famous underachieving family memebers that you can bring up? Maybe you can try the angle that you agree that she does need to learn to function with deadlines, and tolerate a certian level of 'unfun' but that 'poor fit' may be preventing her from having a chance to learn at her readiness level.
Summer programs can help a kid in this situation quite a bit. I imagine that being surrounded by other like-minded kids who enjoy learning for learning's sake must be wonderful.
If nothing else works, just be there for her, and ask her what her goals are. Read: 'what high schools won't tell you' and ask her to also, so together you two can start planning a journey to college and adulthood. Maybe her parents would enjoy the book's achievement orrientation as well. You can take trips to colleges together. I think that at this age, the name of the game is 'Motivation.' What would motivate her?
Love and More Love,
Grinity