Right, I've had a few really bad experiences with garages and the like, and having learned the hard way, playing it nicey-nice is not the way forward. He needs to get the big guns out straight away.
If its genuinely as bad as you say, I would phone the local police station and ask their advice. They'll presumably fob you off but they might give you the ombudsman contact number, and if you can get an incident number all the better.
Next I would get in touch with a solicitor and register your interest in a possible case. Then the next day take a letter round to the body shop saying that you have a) contacted the police, b) contacted the ombudsman, and c) will be taking them to court.
They'll probably say "whoa whoa, we'll fix it", but in all honestly, would you trust them again?
In short, if you genuinely think they have done it seriously wrong, you will win damages. If you can't proove much, you might not win. But in my experience if a garage is that unprofessional, they won't do sweet FA unless they are told to by a court.