How big is she that you don't think she will fit into a 15 inch saddle for short amounts of time? ( Maybe she won't, I have no idea, just asking.)
I weigh a little over 200 lbs and I own a 15 inch saddle that I could ride in if it was all I had. Now I'm more comfortable in a 16-17 inch for sure, but if I was just a guest riding for a short amount of time I don't see why I couldn't ride in it. I keep it because I figure it's a good spare, I wouldn't get much money out of it, and one day I might end up with a horse that it would fit perfectly.
On a side-note, how do you like the Reinsman Flex Tree saddle? I had one briefly (in a 17 inch) and I loved the quality but it gave me back pain. The lady that bought it loves it.