I think they should pass that on to the school and the behaviour worker. Give them the whole situation and what he said, and explain they they are worried that he is learning that negative behaviour. Sounds like they are doing what they can otherwise. But anything like that, anything at all, needs to be communicated immediately, that way the behaviour worker and the school are aware of it and can help his parents to teach him positive behaviour. Good luck to them, what an awful situation to be in.