I would with a crop sensor. The 28mm is a great lens with very little distortion but it isn't really wide per se unless used with a FF and then it's the equivalent of a 24mm in a 1.5 crop body. The 24mm of course is an 18mm equiv. on a full frame.
Wait, are we talking DX or FX lenses? If we are talking DX lenses, on his cropped sensor, whatever the mm rating, it is what it is. If it is a FX lens on a DX camera (his D80), it is the other way around

... that 24mm turns into a (24 X 1.5) 36mm and that 28 becomes a 42mm. Neither very wide. I would suggest that if width is his priority, the Sigma 10-20mm is the best choice, but if low light sensitivity is more important than width, the Sigma 18-50 EX DC HSM F/2.8 is the better choice.
Honestly though why don'[t you look at a Tamron zoom? The 17-50mm 1:2.8 covers everything you mentioned and more..
Tamron | 17-50mm f/2.8 XR Di-II LD Aspherical [IF] | AF016N700
I would counter with the Sigma lens because it beat the Tamron and Nikkor 17-55 3 times out of 3 in a shootout last year by 3 independant sources, plus you get a free 3:1 macro tossed in on top of the deal.
Uh oh... we're adding options, not taking them away... lol
Sincerely, the demands of a landscape and portrait lens are vastly different, and though we can take pictures of people with a sub 50mm lens, its just not the same quality and final result as a lens between 85-105mm.
Decisions, decisions!
(for Wandering Pugilist): BTW, did you go to the link in my first post that I provided at the top? If you did, you would have seen that... well... neither prime lens is all that great, both suffer from distortions up the wazoo (both well over 1%, which is pretty significant for a prime!).
Let me give you the direct links and save you a few seconds:
Nikkor AF 24mm f/2.8D - Review / Test Report
Nikkor AF 28mm f/2.8D - Review / Test Report
Now, I do not know your budget... but that Sigma to me looks better and better all the time. At the 18mm range, the Sigma has distortion, but less than the 28mm does (it is the better of the 2 between the 24 and 28, BTW). By 24mm its near invisible, and from 35mm up, the lens is basically free of distortion.
Again, let me save you 2-3 clicks:
Sigma AF 18-50mm f/2.8 DC EX macro - Review / Lab Test Report
Now, just to let you know, they preferred the Tamron over the Sigma, which to me is hard to believe based on the tests made by the 3 independent photography magazines back in early 2008 who all chose the Sigma over the Tamron and (3 times more expensive) Nikkor.
Edit: Ok I did some research. The test on the Sigma 18-50 links above were done with the
2nd generation Sigma, which was not as good as the
3rd generation one (the one that you want and the only one that is available new today... the 1st and 2nd gens were discontinued when the 3rd generation was released). You can identify the new Sigmas as the 18-50 DC EX
HSM MACRO F/2.8 (first gens were pityful! QC issues, back focusing and poor performance). The distortion is even less on the HSM version as well as an increase in sharpness, reduced CA characteristics and better flare control. Of course, it has the HSM, meaning that cameras like the D40s-D60s can use this lens and still get autofocus to work.
So... how confused are you as to a choice now???
Not that my word makes any difference to anyone here, but I own the 18-50 DC EX HSM, and have been nothing but impressed and very happy with the results it gave me on my D200.