I haven't used chips away personally but they were recommended to me by a mate who had some work done recently - Problem is they are a franchise so it depends who you get.
May be an issue with insurance if it was not a public place - Had a problems with that on a petrol station forecourt. I'd have a word with him first through your mutual friend if that's a better approach and see if he is going to do the decent thing!
Good luck with that and I'm amazed you kept your cool, not sure I would have!!