That makes about as much sense as saying it was better for the wild buffalo to only eat ranched cow meat, not knowing they killed off the wild buffalo to make way for the cow pasture. Your friend has some weird ideas.
The only environmentally sustainable fish farming is fully contained tanks, on land, with sewage treatment of the fish waste, and farmed fish that eat a vegetarian diet (not fish meal). Tilapia, catfish etc can be farmed this way.