There's a lot of fallacies on the dialog. Here, with names:
I recently argued with a friend about veganism. My position was basically that everyone who can allow a dietery change in their lives ought to be vegan.
Bad argument. Fallacy: appeal to possibility, meaning that if something is possible, it is mandatory. The possibility to change diet is not an argument to do it, like the possibility of voting Hitler is not an argument to do it.
I argued that these days there are sufficient replacements for the nutrition provided by meat, and that there is no difference in the worth of an animal's/human's life.
Irrelevant. Fallacy: ignoratio elenchi. The argument is not related to the problem. Using your right hand or left hand to vote Hitler is irrelevant to the decision of voting him. One or the other forms of nutrition would be good, but that does not justify the decision.
However, when they said that one justification might be that humans are at the top of the food chain and that they are the most advanced
Fallacy: questionable cause. It is not because Hitler is the head of the government that he must kill his people. It is not because your friend is at the top of the food chain that should kill a specific kind of animal.
They also aknowledged that in a perfect society where the killing of animals is completely unnecessary, it would still be okay.
This is a pure rhetoric fallacy: it does not provide an argument. The form of the proposition is "in society X, killing animals is ok". Why? If we assume that a previous argument is true (e.g. killing animals is Ok because we can), this is called an stacked fallacy.
Following, a discussion of possible arguments.
- Killing animals is not good or bad per se. Good or bad depend on the purpose. If meat is tasty, it is good. If meat is expensive, it is bad. etc. But any interpretation of a final good or bad truth is subjective.
- The argument that animals suffer is a humanization of the problem. I'm not with or against animal suffering, but such is an inconclusive and long philosophical debate.
- If we assume that the planet has environmental issues (which is an idea that a lot of people reject), then you might have an argument. Google for "environmental impact of meat production" for formal data on the matter.
- If you know that eating meat has a negative impact on health, you might have an argument. But this is also a debatable idea.