Hello; Buying and operating a heater for an outdoor pond durig a typical Ohio winter may well cost much more than a 75 gallon tank.
Does the pond have an inlet and outlet stream/flow? If so, the issue may also include the chance that some non-native species will move into the surounding waters. This may lead to more than how to save the fish as you do not actually know what was placed in to the pond. Some might argue that this situation will need the input of a local wildlife agency. It is possible that if you take that approach the entire pond might be poisoned and that the neighbors may face consequences. Should you knowlingly allow the situation to continue it may be possible that you can assume some of the responsibility. I understand that you have sympathy for the neighbors, but they have placed you in a pickle and apparently dumped the fish without your permission. Good luck.