Let me preface this by stating that any modern dSLR from the last few years is capable of taking excellent photos and the limiting factor in quality is likely going to be the photographer. So, without question, the Canon Rebel Xs is going to be able to do the job until she grows into something bigger.
Anyways, as mattdm mentioned, the Pentax option is likely the K-x, which is very highly regarded for its low light, high ISO, performance. Like the Sony option, Pentax is stabalized in the body and so any K-mount lens attached to it benefits. One thing that people often miss as an advantage of Pentax is the availability of some extremely high quality used lenses on the market as Pentax has a very long history of backwards compatability and you can put any of their long history of lenses on their cameras (m42 require adapters, mind you). As a K-5 shooter, I'm obviously partial to the Pentax line, but a quick Google of the K-x will demonstrate that my small amount of praise is the minimum you'll hear for this camera.
The Nikon world offers up the D3100, another excellent camera. Like the K-x, the low light performance is very good, but I'd say the biggest advantage it offers is for video buffs as it will autofocus in video mode. Going Nikon shares much of the same advantages of going Canon: lenses, substantial growth path, and the like. Anyways, the D3100 looks like an excellent piece of kit.
However, if she can get a good deal on the Canon Xs, it's a good camera to get rolling with. A little dated now, maybe, but it still takes a fine picture. The Pentax and Nikon suggestions are much newer and a little more feature rich, but will probably cost more and I don't know her budget. Regardless, I'd suggest that she head into a camera shop and try the options out, looking at things like feel in the hands, menus, speed, rear display, etc. Once she's handled a few, she may change her mind on what she ends up wanting.