JP, I shoot Canon, but the principles are the same. FWIW, I have a set of very decent tutorials from Michael Andrews and one of the DVDs is about accessories, including lenses - more about that later.
From your description above, it's hard to tell exactly what kind of photography she will be mostly doing, which of course will help dictate what lens would be best. There are hundreds of lens choices, but the general types would be dictated by the types of photography she'd be doing. For example, if she was into shooting sports and wildlife, then a good telephoto (or zoom) would be in order (say 300mm or bigger). As a general rule (not always hard rule), prime lenses give you better image quality (IQ), although in recent years, lens manufacturers are developing zooms (variable focal length) that have IQ nearly as good as the prime equivalent. If her interest in landscapes, then look for a good wide angle (like 15-35mm zoom). For portraits, you want a decent prime lens of 50mm - 85mm (or equiv). Since that camera is a "crop-frame" (DX) sensor and not full frame, it is important to remember that the "effective" focal length is going to be about 1.5x greater than if the same lens were sitting on a full frame camera (Full frame means (basically) that the sensor size is approximately the same as 35mm film camera. A crop sensor has a smaller sensor. So, for example, a 50mm lens would look like an 75mm lens on a DX sensor camera (more telephoto effect with respect to field of view). This is why many sports and wild life photographers these days opt for crop from sensors - they get more "reach" (my 300mm prime is like a 420mm )(The crop factor on Canons is about 1.6x)
With all that said, assuming you want to get a good "all around" lens, I will tell you one that Michael Andrew highly recommends and a good, light, versatile lens, which is available in a Nikon mount. That is, the Tamron 18-270mm zoom. It's a decent lens that is not super heavy and has decent image quality. Now, I have not used this lens myself, but he really sings its praises as a good, inexpensive, all around lens. And, it gets mostly good reviews on Amazon. With 18-270 focal length you have decent wide angle, all the way to decent telephoto capability all in one lens. 18-270 on her 1.5 crop sensor would be about 27 to 405. That would give her a lot of versatility.
Here it is on Amazon <<
Basil