How does he plan on getting to the car in question? Could he not just go pay for it, go to the post office to get the tax and then collect the car?
Thats what I've always done as I've normally needed to get a lift to collect a car, so I just get a lift to the PO too - they're everywhere so it wont be far out the way.
I'd always advise to do that but I had to go against my own advice when I bought my current car due to location and timing reasons. I printed out the forms and had them filled in before collecting the car and drove off straight to the post office. Sods law, I get pulled over. He asks about the tax, I explain that I had just bought the car (showed him the receipt with time on it), was on my way to the post office straight from the place I bought it from (my location confirmed this) and showed him that I had all the documentation including forms already filled out. He sent me on my way and wished me a good weekend, I did likewise.
Sadly, there's plenty of idiots about who would take the attitude of "Oh noes hear comes teh pigs, i shud act like a tw*t to get out of this" who wouldn't have gotten away with it.
At the end of the day, I doubt the police are that bothered if you explain the situation - the system will pick you up quickly enough anyway when the car gets registered to you without tax/sorn