Does this help?

http://www.naturfotograf.com/lens_zoom_03.html#AFS70-200VR

AF Nikkor 70-300 f/4-5.6 G? I own one, and it is quite poor optically
and mechanically, especially above 200mm and when used wide open at
all FLs. I don't own a Sigma 70-200 f/2.8 APO, but I have heard it
described as good to excellent, and it is cheaper than good-quality
Nikkor zooms. I own another Sigma APO lens, and its mechanical
construction is reasonably good (some people don't like the matte
black finish because it gets dirty easily, but it can be cleaned).
Besides giving you an image quality that likely is visibly better, the
Sigma also gives you one to two more aperture stops, i.e., 2-4 times
faster exposure fully open, compared to the Nikkor 70-300. This means
a lot for night shots. With a Sigma 1.4 teleconverter, you would get
up to almost a 300 f/4, still faster than the Nikkor 70-300 and
probably better (or at least not worse) optically than the Nikkor.
Weight is another matter entirely. Your wife might not be able to hold
up the Sigma for hours on end, so a tripod or monopod would be useful
(unlike the Nikkor, the Sigma 70-200 has a properly placed, removable
tripod mount).

I cannot comment on the Nikkor 28-80, but the overlap with a 70-300 or
70-200 is too small to be of any importance. It is simply a zoom with
different uses.

I have that lens on my 300D/20D and I am very happy with it.  I do high
school sports and nature photography.  A little soft at 200mm but not
unacceptable.  Works nicely with the Sigma 2x Teleconverter.  A bit of a
beast in weight, but I almost always use a tripod or a monopod
affixed at the lens collar...

Thanks everyone for your input.  The Nikon lens would be VERY nice but can't
justify the extra cost at this stage - she's not a professional
photographer - yet!
Seems like it would be a good choice to start the ball rolling and give her
some encouragement.
Thanks especially to Deedee Tee & Gambo for your constructive input.
Have a great Christmas and New Year too
Hew

 Yes, IMO and IME she would get a *lot* of benefit from the lenses
with larger apertures. Larger aperture means a faster shutter speed can
be set, which will stop the action of sports much better.

 There are a couple of different versions of the Sigma f2.8 lenses; I
would suggest the HSM version of the 70-200 f2.8. I've just gotten one,
and got quite decent results at a dog agility competition with it this
weekend despite shooting without flash in an unlit quonset hut.

 If you look at getting her a WA to short zoom (e.g. the 28-70 or the
like), read the specs carefully; there is a Sigma lens out there which
is advertised as being f2.8, but does not have a *constant* aperture.

One caveat, though: the 2.8 lenses will be *much* heavier and larger
than the G lenses. You might want to consider also getting her a tripod
or monopod to use with it, especially since she shoots at night.

A 2.8 will be a huge advantage.  I have the Nikon 4.5/5.6 ED AF-D (non
G version) and it is a better lens than the G (and that is not because
the other is G, it is because  Nikon built a less expensive version
which isn't as good).   I only find a problem when full zoomed too 300
(looses some contrast).   Other than that I like it given its size and
lack of weight  I still used it when I don't want to lug the large lens
around . However, I bought the Nikon 80-200 F2.8. AF-D  I have never
looked back. It is a superb lens.  I don't know who it compares with
the sigma, but the F2.8 will do here well at night.  Autofocus will be
faster and the faster lens makes it easier to compose etc.

At  BH Photovideo  the Sigma looks like it is  about $750 and the Nikon
looks like its 819.00 after rebate.. you choose!