Thanks for the insight and the links. That Sigma 100-300mm looks intriguing, but I don't think she's ready to drop a grand on a lens yet. Thing is, the last time she was in Africa (working), all she had was a crappy 25+ year old Pentax film SLR with an equally crappy 28-200mm lens and her pictures didn't turn out so well.
I've been looking at some sample photos from both the Sigma APO and Tamron 70-300mm and neither are terribly sharp at longer focal lengths, a fact which their MTF charts corroborate. Personally I wouldn't be satisfied with their optical performance, even if it's only being mated to a 6 Mpixel sensor. You get what you pay for I suppose.
That leaves the Nikon 70-300mm VR, which according to the linked review has, despite some CA, quite decent optical performance and stopped down a little is still sharp at 300mm. I think I might try convincing her to go that route. If not, I'll point her toward the 55-200mm VR
which received a very favourable review.