OK, if the pond is on the guys property and not connected to any streams or anything, theres nothing wrong with adding goldfish.
Because someone "owns" their property, that does not mean the property is no longer part of nature and therefore it is ok to do what you please with it, the concept that we are trying to explain is that an introduction of non-native species effects every other species for hundreds of miles(and eventually over the whole planet) some more than others, the introduction of non-native fish into the pond will even effect land animals hundreds of miles away
For example, (Note* just a quick example) the introduction of goldfish into the pond could transfer diseases and parasites that are not currently in that pond, the parasites/diseases effect other aquatic animals and amphibians living in the pond, the populations of the pond are decimated, meaning the food source of some land based animals is depleted, land animals in the are feeding from the pond must move to a new area where they must compete for a food source with an preexisting native population, that native population is destroyed because the invading population is a better food gather/ hunter/ killer, that native population that dissapeared was the food source for other animals, for example, birds, those birds don't have a food source and die off, then there is no longer a population of birds migrating to a certain area at a certain time of the year where their main food source would have been a certain bushland animal, that bushland animal population, now having no predator, would then grow larger, and need to feed more, and would eat too much of their food source, say for example the animal in question eats seeds, say they eat the seeds and through their feces spread the seeds where that plant reproduces, but since the population of these animals is increasing, they must move to new territories for room, and thereby spreading the seeds of the plant into non-native areas, where it thrives and outcompetes the native pants, and so on and so on
the possibilites are endless, and that is why we should not be putting non-native species in habitats, even just one