No,
I would object, especially as the council must have known that some motorists had prepaid, so therefor would not see the notice on the meters,
i would argue that a sign should have been placed where it could clearly be seen by anyone parking in the restricted area,
One may have been placed there, but maliciously removed by someone wanting to cause trouble to your husband, especially if he parks in the same spot every day,
you may get nowhere with the argument, but it's worth a try,
if only to get it off your chest by telling them what you think of them.
Good luck.
pete.