I put 2 flatwater kayaks on my roof all the time, and drive on dirt roads (though, the dirt roads I've been on with a load were barely worse than a paved road).
Anyway, I agree with Kevin. A good coilover (like my Tein Flex) would work just fine. I would hesitate against recommending springs, but only because its hard to find them without lowering. I would also suggest upgraded struts. But, I would also warn him that the Tein Flexes will make the ride harsher. I used to like it, then it pissed me off, and now it doesn't bother me any more. Oh, and they may lower him a bit too much depending on how bad the roads are that he drives on.
Sway bars actually will help quite a bit in reducing mushiness, but it won't really help with the extra load in the back of the car.
