Short of building up the Soma with weight in mind (which might run you over your budget), you might consider getting a bike like the Myka and making changing out few things to bring down the weight, obviously starting with the fork and wheels. It's not the most cost-effective option, but as I know from my own short legs, sometimes small riders have to make compromises others don't. This seems especially true in the MTB world. There are seemingly more options out there if she wants a FS bike, but with that, she'd likely have to up her budget if she wanted to get something light.
Depending the used MTB market in your area, you might keep an eye out for a FS bike on Craigslist. Or post something on MTBR's Women's subforum; it seems like there's always a person or two trying to sell a bike. In
this thread alone, two women are trying to sell their XS FS bikes.