Personally I wouldn't buy a gallon size, unless I was looking to only feed 1 or 2 relatively small fish for awhile. I would think it would be ethical, so long as normal precautions are taken to keep the fish alive.
That's very correct, the number of fish you're planning on breeding in the pond would be a big factor in determining whether it's wise to go for it or not. Perhaps, he'd have intention on small projects for it, so it's a good decision if that happens.