I think both were in the wrong here. But the cop more so.
#1. If you are going to pass someone,then go faster than the vehicle you are passing is going.
#2.If the vehicle you are following is doing the speed limit,then why pass them?
I would go to court on something like this. Just because a person is a cop,it does not give them the right to tailgate. Even if that woman would have slowed down,and used her turn signal,then got behind the truck,who is to say the cop would not have given her a ticket?