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the Bilstein 5100 series is rated for heavy duty applications, but the shock won't matter much once everything is settled. Heavier weight cannot be offset by shocks, you need more spring rate.
Look into the Timbren line of bumpstops to improve spring rate for heavier loads. they are expensive, but if your hauling something heavy, this is the way to truly offset it.
if your shocks are worn, however, you may need both.
Not that much heavier, they only add another 800 lbs of capacity. And, they add from 3/4-1 1/4" of lift, depending on how worn the stock springs are - if he doesn't want to lift the rear, then the Z71's aren't an option.You could also put the Z71 springs in the rear which are heavier duty spring.