Zoom lenses are notoriously low aperture by at least 1 stop, usually 2 or more (20x zoom lenses, ugh!), compared to prime lenses, which are always better for low light conditions. Now sport photographers need to freeze action, hence the higher shutter speeds and thus large apertures needed.
Now instead of using a 300, 400, or 600mm f/2.8 lens, on a tripod or at least a monopod, can't she get closer to her subject?
That would be the easier and cheaper way.
For portraits a 100mm f/2.0 lens is great, but I often used a 135mm f/2.0, wide open, especially for model close-ups. Sometimes longer... it has all to do with viewpoint/perspective, the distance to your subject.