The only "backup" that the finance company will give him is if it was mis-sold, had a major undisclosed fault or is not fit for purpose.
As its a new car, any faults will be covered by warranty and, as he test drove it, there is def no chance of getting round it any other way. Even if he didnt TD it, he would still be stuck.
As he signed the finance docs on the premises there is no cooling off period. If he had signed a set of "distance selling" docs off the premises then yes there is.
If he is looking to get rid of it then the most he can expect to get for the car is -
Initial price paid (NOT list price, but price he paid)
MINUS the VAT (as dealer doesnt get this, 15% of the price he paid goes to government)
MINUS around another 5% for depreciation as its classed as a "used car" now.
So, in general he is looking to get back around £750-800 for every £1000 spent depending on model etc.
Edit - note, this is not general selling/finance info I have given, its direct from the car trade....
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