well .... it won't hurt to seek legal advice to see what his rights are ... end of the day unfortunatly he made a mistake ... but no-one is perfect and we all make them.
personally I agree with part of what lwells said. why did he buy if it seemed dodgy. even if I was on a budget, I'd rather save some more and wait, rather than rush and buy a potential cut-n-shut. anyway ... that aside, no one "deserves" to be ripped off. (that was a little harsh I thought)
get your friend to go see a lawyer, or if he doesn't have money then go see LegalAid (look it up in Yellowpages or something), and see what you can do. Also speaking to the Fair Trading Commission via the phone wouldnt hurt (again to see what your rights are and what the responsibilities of the dealer / seller are)
Was the seller a "registered dealer", or was this more of a private sale? it will change the rights of your friend and the responsibilities of the selling party, depending on the terms of the sale.
Good luck to him. I hope everything turns out okay.
PS. and as far as the PRICK who banged him on the side and drove off ... that makes me very very VERY angry. Someone could have been killed and he drove off. I hope he gets what he/she deserves.