Oooohh....23.5 Mondo. That's small. I basically wear the same size and it has been a very real problem for me as well. So you're right, it IS much harder to turn a board when your feet don't reach the edges - this is especially true for toeside turns.
Basically there are NO boards out there that will be narrow enough for a boot that size. I have found that the solution is a torsionally soft board that is easier to turn. For me, even mid-stiff boards are too stiff.
It may help to put her on a shorter board as well, since waist widths are narrower on shorter boards. So although a 150 cm board may be ideal, she may find that a 145 NS Infinity (for example) would be stable enough for all-mountain riding, but short enough to manage.
From my own experimentation, I have decided that the Never Summer Pandora is the best "park" board for all-mountain riding. It is very damp, but is still soft and playful. I picked one up this year and I'm loving it.
The Capita Space Metal Fantasy (female version of the Horrorscope) has a very narrow waist and is also a very fun board.
I also owned the Gnu B-street. Great board as well, liked it more than the SMF, but found that it really bounced me around in choppy/rough conditions. The magnetraction is great and it floats well in powder.
How is she doing with boots? Salomon makes some of the smallest sizes out there, if you need some options. Wearing a stiffer boot and binding may help her reclaim some of the responsiveness she'll lose on a softer board.
I hope this helps. It took me quite a bit of experimentation to get the best set-up.