I know business is hard to come by at the moment but if your customer is not going to get the right truck for the job I would let someone else look after it, as you could be brought into any legal action depending on what you did / advised or the nature of the accident.

The only "person" who can advise on a capacity upgrade is the manufacturer as only he knows the ultimate strength and design capabilities of the truck and components. If he says "yes" do "XYZ", as long as you do "XYZ" you are in the clear. If he says no then no it is. It is highly probable that you could add ballast to the truck and all will be fine and dandy till something goes wrong, but its too late then.