Don't get me wrong, if I ever get involved in anything like this, the finest of fine toothcombs and the largest of magnifying glasses come out and NOTHING is allowed through. If it can't be easily accessed, tough ****, someone needs to start disassembling things. If it can't be checked, it doesn't get OK'd, simple as that.
I just think it's a bit short sighted flatly refusing to get involved, often your customer hasn't done anything wrong and acted in good faith. You can get highly moralistic about it, but I'm not prepared to lose good custom for the sake of (in my opinion) misguided morals. I will report the miscreants when possible, and it will be a high price that goes in to sort the issue, but you should be able to ascertain the safety of any installation and if you recommission it, where's the problem?