It is a great supplement. Indians, excuse me, Native Americans used it centuries ago. I use fish emulsions and I'm sure that whole fish would be a great supplement, maybe better. You could use fresh water fish which would have less salts and lower fat content than salt water. I would suppose that your growing outside since your looking at using fish. Fish stink, fish emulsions stink. You'll have more critters in the mix that you'll want to deal with. You'll attract every thing from field mice to dogs to possums and skunks. I don't worry about it since i'm inside an I have a can filter so no one is smelling it outside that room.
Given you friends theory then if you use bat guano then your weed will taste like bat shit. And I guess my tomatoes taste like cow shit. What about my carrots and onions, they should really should taste like oak leave and worm shit. It sounds good, its just not right.
If you had some where to compost it,it would be great after a couple of years. If you put it in a bokashi mix it would be useful sooner. But safely and effectively using fish would be a lot of work.