I guess you missed the part where I said, "there would be no problems were he not off roading it."
In reference to the OP's information that the rider in question was using his bike on road...
"He used to weigh 280lbs and the bike didn't have a problem with that. It was ridden hard (on the road) and has done around 10.000 miles without a single problem."
You won't find me giving advice very often, (and I am admittedly out of the loop with the very latest tech), but I still know what I know!
There would be absolutely no problems with those hubs riding on road at four hundred pounds.
Indeed, the bike is only used on the road, and with this weight he can only ride it leisurely. I have yet to see him exceed 12MPH since he started riding it again.
I've swapped the balls in the Altus hubs with better ones, but the bike hasn't been ridden since I delivered it. Once it's covered 50 miles, I'll get over there to have a feel. Then I'll gradually increase the intervals as he loses weight.
I've been offered a set of Sram X-7 hubs, and those seem to be pretty strong. If the new balls also fail, I'll take the wheels apart and swap the hubs.
EDIT : Tandem hubs are an option indeed. Most tandems are designed for that kind of weight.