Pop eye is usually caused by an environmental problem or bacterial problem...
Environmental problem would be a build up of gases in the water normally nitrogen but not always. I would clean the water frequently and this should help the problem...if that does not work it could be bacterial. In that case I would search out a medication that treats pop eye...most times simply improving your water quality can get rid of the problem because most times it is not bacterial.
remember perhaps the pop eye could also be a symptom of a more severe problem, it is often seen with swollen abdomens as a sign of dropsy in which case it may or may not go away.
NEVER freeze your fish in the freezer for euthanization it is inhumane the blood crystalizes before it actually dies, imagine how painful that would be. I would not think of pop eye as something to euthanize a fish over it is normally quite treatable.
More humane euthanization would be to use a lethal dose of clove oil it is an approved method for anesthetizing fish. It has a similar effect as when vets put a cat or dog to sleep. But you should know how to properly euthanize something before you try imagine doing such a thing if you did it incorrectly, that isn't fair to the fish. I have experience with it through my studies but I would not recommend an inexperienced person to do so without proper research and instruction.
Hopefully his pop eye gets better