Pacu would be far more palatable than tilapia I think, given their natural diet. And really, THAT is what is key to achieving a tasty fish--proper diet. No goldfish, no pellets, whole, natural foods, including bugs if that's what they eat.
There are a few things to remember about cold water (fish/animals included) vs warm. Higher activity and metabolic levels in warm water coupled with lower O2 levels are the main issues.
The young lady should know this #1 rule of ALL fishkeeping--ANY fish will put ANY OTHER fish in its mouth if it thinks it'll fit. Period. The fact that a tilapia is a cichlid has not a thing to do with what it eats, otherwise how would she explain the Geophagus family of Central American cichlids? How would she explain the many algae-eating cichlids in one of the African rift lakes (Malawi, Tanganyika, Victoria)? How does she explain discus, which eat invertebrates mainly (and beef heart in the home aquarium)? Cichlid does not automatically equal piscivore.
She's essentially correct regarding fish waste. Other materials may release ammoniacal/nitrogenous compounds, depending on what they are. E.G. starting a new system by using fish food left to decompose, bits of fish, invert flesh (shrimp, squid, etc), that sort of thing.
My question to you is will plants grow well in a system that's in the 50s? I have read often that there's a limit to low temps and a plant's ability to actually utilize nutrients present.