Any Pentax mount AF lens that doesn't specify HSM or SDM only (emphasis on the Only), will work with your girlfriend's camera. Actually they all will but she will be stuck with Manual Focus on those types. Nothing wrong with the idea of upgrading the camera but she will still be stuck with the kit lens (and it may even perform worse due to the higher resolution of the new camera). The Sigma 17-70 non HSM is a decent lens, I had one once. If she's use to the 18-55 focal range, I would actually suggest something like the Tamron 17-50 f2.8. If she doesn't use the wide end very much then maybe the 28-75(?) f2.8 would suit her better. If you can spare the extra cash, a DA*16-50 f2.8 may (optically) be the best of the bunch (having never tried the tamrons, can't say that for certain). I Can say however, that side by side with the older Sigma, the Pentax blows it away. The Pentax is both SDM and Screw drive AF but will only use SDM on cameras that are equipped with it.
ALL of that said. If you want to consider a new camera, why not a K-x with the 18-55 and a 55-300 kit? The newer kit lenses are much improved over that original one (though still not great) and the 55-300 is said to be one of the best consumer zooms in that range available.