This is how I coped, maybe it will help :)
Go to a GOOD salon and have it thinned. (aka layered)
You have to be careful during the cut, or she will come out with a bob cut. I learned that the hard way. I now stay with the hairstylist and monitor how much they cut off, but the layers help a lot, and cut it to just below sholder length. Still long hair, but easier to manage, lighter to carry and does not hurt to brush.
Be careful of the neckline of her clothes. Every time I wear my wool coat, I have huge knots at the nape of my neck, so I wear a silk scarf over my collar to keep the knots at bay, however, turtlenecks cause serious damage even if I wear my hair up. My daughter keeps these out of her wardrobe.
You can also switch to a silk pillowcase at night to help keep it from knotting. We could not afford that so we use terrycloth ponytails, gathered in sections down the back of the head, while the hair is wet and brushed, to keep it from getting knotted during the night.
My daughters favorite now is a loose braid at night. Keeps her hair still wet by morning (so thick) so it appears she is freshly washed and can still use a styling gel. I must note that now she is 17, and finally, sometimes, styles her hair. She is still a wash and go type, she is pefectly happy with that (it is not messy, but no effort went into it). I was more concerned about my hair at her age, but she is beautiful and I love her just as "granola" as she is.
It is a long road, but you both will get there!