The correct way to euthanize a fish would be an overdose of MS-222 I believe. I know in our aquaculture dept, it is considered inhumane to freeze an animal. Regardless of species.
If it cannot survive the cold, then there is little chance it would have in that area. Dependant on water column temperatures. However, if released during summer, it could stand a chance to cause some havoc.
Let him keep the fish. If he says it dies, make him produce evidence of the body or have a biologist confirm it. Let him get a permit for it, that way big brother can stop weeping.
Sad. I too am waiting to see what comes of the python ban.