Did you ask the rep how to handle the deposit. I am totally sure they will help you work a way to ensure that they get the money without embarrassing your nephew of course. But yea...Im sure not all of it is going to the vehicle. I know I would def be getting myself some drugs with at least a little of it. But that's me. Its hard to say unless you see with your own eyes what the deal is.
But any money in an addicts hands..especially if they claim to be newly clean is asking for trouble IMO.