Trick .... RUN AWAY!

There's no profit in being dragged into a dispute between thieves. Tell the customer that this job is outside the scope of your business. Send him a BIG bill - his failure to pay it will make him look bad, should he continue to try to drag you into this. Answer his inquiries with "I'm billing you for my efforts so far, there is no resolution, I have no idea what's going on, and am not comfortable proceeding."

I've seen far too many marginal customers seek out marginal contractors - whence the two each try to take advantage of the other.