Work it backwards.
Get the stock to fit here first, then without adding weight to the butt stock, length/contour the barrel so the rifle is balanced.
Right around 22" is the perfect barrel length for harmonics, while a longer barrel will give a touch more speed for longer shots, and a shorter barrel will make the rifle handle much faster for running shots instead.
Bluntly, build the rifle to fit her so she can use it for free hand standing running shots when needed, and let the chips fall where they may.
So on that note, would start with a 22" barrel, and see what the barrel needs to be profiled to balance the rifle out (so the weight of the rifle is centered between her hands). If you get to the point that you are pretty much maxed removing more metal from the profile of the barrel to balance the rifle out still, then its time to start to shorten the barrel instead.