Try to talk to her if she runs away ride your horse, if it starts to get nasty talk to her parents.You could also tell her "you don't feel comfortable riding her horse in case something went wrong and the horse got hurt", or something along those lines.
Are you sure its not worms.... Chance looked VERY fat at our old barn - so we cut down her feed and down to 1 flake a hay --- but her ribs her showing but she had the "hay belly" Then I realized she wasnt getting wormed at all So I took it upon my self to do it, and she lost the "hay belly"
If its a 17y.o doing the barn stuff... she may get lazy or forget doing the worming -- just a suggestion.
While excessive lunging can be deterimental it certainly is a form of exercise especially when someone is unable to ride. Here is a quote from a horse advice website and there were many concurring posts : Lunging your horse has its advantages. It teaches your horse manners, gives your horse exercise if you are unable to ride, and if your horse likes to buck when first taken out you can lunge him to wear him done a bit.