Why does fish broth made with frozen fish lack flavour? I do not know culinary terminology; so I use 'piscine' to mean 'with a positive flavour of fish' (because 'fishy' connotes negativity).

My friend drank some flavorous fish soup with a milky white colour at  his favourite Cantonese restaurant, one of whose chefs recommended using a fish called 黄鮨斑, which a laborious Google search revealed as yellowfin grouper (abbreviated as YFG). 

My friend could not find live YFG to purchase; so he bought some frozen heads. He pan-fried/seared one head which turned a golden brown, then simmered it (over low heat and an electric oven) in around 4 litres of water for around 2 hours. The final product tasted like water (lacked any piscine flavour) and resembled water (lacked the desired milky white colour). The fish itself tenderised and still retained flavour though. How can my friend correct the problems?
I think it's not so much the fact that you're using frozen fish (which certainly isn't the best), but the fact that you've missed an important step: pan-frying the fish before making the soup. I can say that all fish soups I've had have always had the fish pan-fried before boiling, with ginger added, otherwise you just don't get the same flavour.

Another thing is, in a properly made soup, the fish should not have much fish flavour remaining as it should all be in the soup. 

I would recommend retrying the soup with the same type of fish, but pan frying them and adding ginger. If the soup still does not taste great, it could be that the YFG you're buying just isn't very good, and you could consider using a different type of fish.

Here's a link I found supporting my assertion, with more detailed instructions:
http://www.chinesesouppot.com/4-soup-techniques/1195-how-to-make-milky-white-creamy-fish-soup