Drifter, while it can be tricky, it can be done. I have taken on the same mission for my lady several years ago. It must have worked because she is still hunting, got a wild hog with her bow, and she can't wait for opening weekend just a few weeks away.
The "Rule of Thumb" is(boy that thumb must have been a smart guy)for every inch below the 28inch rated draw weight, you should subtract 2 - 2 1/2 pounds. For some bows like my windows, you would subtract 3 pounds.
The rule works the other way too...you add for every inch over the twenty eight. So you could estimate her bow weight at 36 pounds for the 26 inch draw length, and 32 for the 24 inch draw length if I have my math right.
You can go to the Easton arrow chart and check out what it says, or you could try some 1816 shafts cut to 27 or 28 inches. If you have an archery shop near, you might want to have her try some of the kids arrows in that size.
I am glad to see that you are not falling for the overbowed syndrone, I started Samantha out at 40 pounds, she now has two custom made longbows that she shoots, one is 41 and the other, it's twin, 44 pounds.
I would love to hear what happens, keep me posted.