I somewhat agree but you're the first guy to mention one order per customer policy. If that policy exists the OP's question is kind of moot. If that policy does not exist however there is nothing wrong with breaking the orders up at all. The retailer has no doubt determined they are still making the money they need to at that discount and pricing level (which is why it's set up at those levels in the first place) so personally I don't have a problem with multiple orders, I don't see that as taking advantage as long as the 'one order per customer' limit does not apply.
I would guess that if he is placing $2500 worth of orders, which the above example would be 10 x $20 discounts, I bet if he just phoned them and asked for the $200 off he would likely get it. Less paperwork, processing and shipping for the retailer too.