Adam
I would get your mate to look at other finance methods as well. A straight loan with no early settlement charge is the way to go.
Most dealer schemes have front loaded interest, so any early settlement is hampered by the interest. Another thing to consider is that a dealer HP scheme will show the car on HPI. Therefore it he wants to sell the car, in theory he needs to settle the finance first. With a straight 3,4 or 5 year loan he can sell the car and just get a top up loan for the new one.
Make sure hes getting a good rate as well. The Merc dealer tried to con my dad into 11.0% APR the other day. In the end he got 6.5%. Go to a site such as
http://www.moneyextra.comhttp://www.moneyextra.com That will give a good indication a repayments.
If he is looking at finance on a 3 year PCP/Ballon type deal, just go for a straight 5 year loan. He will probably have a smaller settlment at 3 years on it than a PCP.
Dont know if that helps. Using other methods will not prevent him tuning the car.
Jonathan