Right, firstly take a deep breath.
Couple of things. Firstly who is asking for the money – local authority / council or private firm?
Its likely to be a private firm who are big on threats but can do basically **** all.
1) No one take take anything you own without a High Court Order.
2) No one is going to the High Court for £600
3) They will possibly try to take him to County Court, this isn’t that bad IF he doesn’t bury his head in the sand – if they got to CC and admit “yeah, he offered £x a month but we flat refused” they will lose.
4) They won’t be able to take the car if it’s secured against finance, the finance cos take little shit from these firms.
IME with local authorities and councils its harder, they aren’t restricted with the same rules as the debt collectors, but their weapons of choice are 1) garnish salary if he’s PAYE or 2) threaten prison if that doesn’t work, but I’m 99% certain that’s only for non-payment of COURT fines and or Council Tax.
You can follow up with more detail if you like and I’ll help where I can and be honest about whether I’m stating fact or theory – but Legal Beagles is the place to go really.