Since he said the problem arose when he installed the coated strings, I'd say that's a pretty good assumption. A functioning bridge ground will not do anything unless there is a conductive connection between the bridge and your fingers, and tapewound strings are wrapped in nylon (non-conductive). Black Beauties are a bit different because the coating tends to scrape off fairly easily wherever the string connects with metal or wherever it is played, allowing conduction- I would sometimes have this problems with brand new sets though when I used them.
The easiest way to tell if it might actually be a bridge ground connection is to touch the bridge with your fingers- if the noise goes away when you touch the bridge the same way it does when you touch other metal components on the bass, it means the bridge IS grounded correctly and the strings are the real problem.
