My daughter has a Nikon D-90 camera body and two prime f1.4 lenses 50mm and 35mm. She is interested in learning more, and I am curious which of a zoom or fixed telephoto lens would help her best? Is getting a low f-stop lens important in a telephoto lens?
It depends on where she's feeling the limitations. I have a kit made up of 15mm, 40mm, and 70mm prime lenses (on a dSLR with the same 1.5× format as the Nikon D90), and for me, that's just about right. (I'd probably trade the 40mm for a 35mm were I starting over — tough call.) For my style, I don't miss having a zoom at all.
Since she has (and is presumably comfortable with) two prime lenses, she may feel the same way, and would just like to increase the range of focal lengths she has available. For that, another prime covering either wider-angle or more telephoto would fit nicely.
On the other hand, she might want to explore the convenience of a zoom. Having flexibility of framing can remove one-more-thing-to-worry-about from the learning process, and remove the potential need to switch lenses in the middle of the action.
Back on the first hand, though, there is a school of thought which argues that prime lenses have inherent advantages for learning composition. Mike Johnston's Case Against Zooms articulates this view well. The idea is that by learning to know a particular prime lens's inherent viewpoint, that limitation actually becomes a freedom.
Since I use my 40mm most often, I can attest to this: having used it to take thousands of photographs over the course of several years, I can know what photo my camera will make without having to actually put it to my eye. That's very useful, and helps me concentrate on taking the photographs I want to take with the view I want to have.
If you are interested in learning a certain type of photography such as birding, then a telephoto would be what you are looking for. If you already have general purpose prime lenses, a general purpose zoom pretty much achieves a similar goal, just with a more convenient package.
Stickily speaking, if learning is your goal, exposing yourself to as many different types of lenses as possible might be the best. You may want to try a 100mm macro, a 70-200mm zoom, or a 18-55mm zoom. These are all pretty standard and will give opportunities that your current kit does not allow.
When I was learning, I used a 50mm f/1.7 on a Minolta SLR. It was a great learning lens for so many reasons.
It helped learn composition because I had to think while I shot -- I moved closer to or away from subjects rather than simply zooming in and out.
It was bright and fast so I could shoot easily in dark locations
I was forced to concentrate on focusing, especially when shooting at wide apertures
I could invoke shallow depth of field, something hard to do on a "slower" lens
The photos always looked so sharp and contrasty, because the prime lens provided such good image quality
Now, I'm really picky about lenses, and part of it was due to my finding out how poor the quality of a large range zoom could be. I would recommend learning on a 50mm prime any day of the week.
Would a fixed or zoom telephoto lens be better for learning?
There are lessons about perspective, field of view, narrow depth of field, framing, and composition that can best be learned by working with a prime lens and having to actively consider the best shooting position to get the shot you envision. The fixed focal length forces you to move to alter the framing of the subject, but that doesn't mean you have to or even should stop moving to alter your composition if you add a zoom lens into the mix.
There are other lessons about perspective, field of view, framing, and composition that can best be learned with a zoom lens. A zoom gives you the opportunity, for example, to explore how the same subject looks in relation to the same foreground/background when shot from different distances at different focal lengths using the same framing of the main subject. In such an exercise you are zooming with both your feet and your lens in opposite directions and comparing the results!
The advice many give to learn to "zoom with your feet" before using a zoom lens is primarily a warning not to stand in one place and stop exploring shooting angles and perspectives if you were to have a zoom lens mounted on your camera. But just because you are using a zoom lens doesn't mean you can't still alter the composition of your photographs by using your feet!
Can you learn a lot as a beginner using only primes (or only zooms)? Absolutely. But you won't be as well rounded a photographer. Ultimately, I think to be a well rounded photographer you need to have the skill sets to use both prime lenses and zoom lenses in appropriate situations as well as the ability to assess when each is the better choice. Whether shooting with prime or zoom lenses, the key is to avoid becoming stuck in a rut (or in one spot) but rather to keep exploring new ways of seeing the world through your viewfinder.