The main reason you should never release any fish once kept in a home aquarium is the potential of the introduction of a disease unknown to the Native enviornment that could prove disastrous to native waters.
Say for example, the bass he had kept and then released, contracted some disease that had been in his 75 gal tank since who knows what else was in there.
This bubonic tasmanian disease then infects his neighbors pond, where little johnny and his buddies fish, and keep a few to later release in a local lake or river somewhere else.
Now the entire river and where ever it goes is infected, and very shortly humongous fish kills start happening because our Native fish have no immunity to it.
Maybe far fetched, but all these Native fish and wildlife belong to me and you and anyone else who really cares, and i dont want to take that chance, just so someone who is unable or unwilling to properly provide for the fish they take wants to keep somthing for a few months and let it go.