Hi m80
Yes, of course under good circumstances, a longer lens will be better. It really does depend, as you correctly point out, on what type of safari he will be on. If there is a very good driver who understands the needs of photographers, then the car will stop and be a good stable foundation for a long lens.
I was merely making the point that a good, sharp pic taken at 250 or 300 is likely to crop and blow up better than a pic taken at 500 that's a bit blurry. Horses for courses...
I have a friend who went on a safari to the Serengetti and to Ngorongoro Crater, but he had a problem with:
(a) poorly maintained old Land Rovers with bad suspension,
(b) 6 people in the vehicle apart from the driver, so they were all jostling to get good views and the vehicle moved around quite a bit, and
(c) they drove right past several good photo opportunities that he was able to grab with his Tamron 18-250 (which he keeps on one K10D body) and would have missed completely if he had been relying on his other K10D with the Sigma 170-500 on it. The results were more than acceptable when cropped and blown up.
So yes, you're right. It depends on the type of safari, the driver, the vehicles, and the person's budget. And I certainly agree re the monopod and/or beanbag. If all else fails, I've used a sock stuffed with sand and tied off - worked just fine
But be sure to use a clean sock, as it will be right under your nose.....