I'm an optometrist who fits ortho-k lenses. There are now many studies that show that they can slow the progression of myopia significantly. I think it is part of my role to advise parents and patients on ways that they can slow down the rate.
One of the reasons for the concern with myopia, is that high myopia can be associated with early development of cataracts, retinal holes, tears and detachments and several other conditions.
I've never fitted anyone under -1.50DS. As all corneas are different you don't get a completely exact prescription, so a successful fit for me would be between +0.50 and -0.50, which is well within driving standard. Some people do get plano though.
There are long term results. I started fitting about 20 years ago and apart from some people losing tolerance, a small infection risk (but lower than soft lenses) and initial discomfort there doesn't seem to be any long term complications. For many people the effect does wear off at the end of a long day, so if your teen is out clubbing at 2am, they may notice a deterioration and several of my patients have a pair of late night driving glasses in the car.
I hope the eye exam went okay today, if she isn't at a level that is suitable for ortho-k yet, then please do encourage her to get and wear some glasses, but its a great possibility for later on.