I agree w/ Dianlya about seat positioning, I wouldn't use it for adjusting reach. He should look at different stems for that.

Disc Brakes: It's true that if the wheel comes out of true the brakes are not affected. I'm not sure how more likely it larger person is to cause a wheel to come out of true than a smaller person, but the fundamental idea of the brakes not rubbing is correct.

The only draw back I see to disc brakes are price and break in time. Obviously they're more expensive, but I do feel they're becoming more popular right now and the price will drop. There is a beak in period with the brakes so they will not feel perfect when you first get them. You need time for the pad to wear on the rotor and make an optimal rubbing surface. There was a nice article written about this recently in a mountain bike magazine, I can't remember which one right now though...

Overall, I like disc brakes. I think the design is a good solid design that works better than V brake in wet conditions. Once you get past the break in period, in my experience they feel better than V brakes. I currently ride w/ V brakes though, DH has disc.

In saying that though, I would not let that be a deciding point. I would just get the bike w/ the best fit and comfort. If the bike has disc brakes, that's an added bonus, or if the frame is disc ready you can upgrade later. I don't think I would personally not pick a bike because of the brakes.

Another idea: you may want to post some questions on the mtbr.com forum. They have a forum for clydesdales (heavier riders) and they would probably offer you good advice on solid bikes for larger people. They might have more experience with the brake scenerio also.