He's not paying twice the market value. He might be paying the CURRENT market value, but that is meaningless as he's not trying to sell it.
You have to compare the COST of the car (ideally taking inflation into account, but let's ignore that and just call it £8k) to the VALUE of the car when he comes to sell it in 4 years time - lets say about £1k, tops (but it'll probably be closer to £600).
So, in fact, he's paying 8 times the value of the car. Which is even dafter (if that's possible).
I made a mistake on finance when I was 19 too. I bought a 306, and ended up paying quite a bit more for it that I should have. I can't remember the exact figures, but I think it was on the forecourt at £7k, I paid £9k in total after three years, and sold it for £4k. I learnt never to use car finance again. Essentially I was £2k out of pocket after three years (on the basis that with finance or without it would have depreciated the same amount), which is nowhere near as bad as James07!