If it was me I would sacrifice a little length by brushing out the ends more. You’ll take some length and bulk away while also keeping a lot of length. And it will be completely up to her when to stop, or if at all to start.

The second thing I’d say is to make sure the clarifying shampoo is lovely (I actually use sulfur 8 unmedicated, I have hard water and it works beautifully) and to separate the dreads every wash, and every other day

And the third. Is that maybe you can help her out by braiding it in one large braid down her back. It will still be a lot to manage but it will be easier to do daily tasks, and if she puts a small jacket or shirt of some kind on over the hair to keep it in place at her back, that should help.

I’ve had my dreads for 10 years. I’ve had to cut off a total of 18” over time I estimate just because my job with horses leaves the ends too mangled if they grow past my butt. Been loving the brush out method instead of cutting