Was the speed limit signage still in place? If not then as long as he can substantiate that then he should be home and dry. If "just" the signage is left and all the works have gone I'm not sure what the legal situation actually is.
I've had a quick punt around PePiPoo but can't find anything. Can I suggest you add a post there which might jog the memory of whoever advised on any previous cases, or else have a look through
this thread which lists cases won with one sentence summaries and references to the detailed threads. You might find it that way.