I was recently in a debate about veganism with a moral anti-realist, and I realized I didn't have a good reason to give him to persuade him to go vegan. He argued that all a person's morals are simply constructs they create to fit their actions and upbringing, and that nothing has an absolute moral value.
Depending on how you define it, this is always true. At some level, moral values are constructed, even if you don't think they are. They can be constructed by your basest instincts and emotions, though, which is almost the same as moral realism. However, the universe doesn't care about morality. And if you get your morality from faith or God, then it's not absolute, because it depends on that faith. Any person's definition of morality can always be reduced to the set of factors from which it was constructed.
Therefore animals have no moral value, and veganism has no moral basis.
... that doesn't make this statement true. The lack of an absolute morality in the strictest terms doesn't mean we don't have something that works effectively identically. For instance, the moral prohibition against murder is something that we created, technically. We base it on fear, empathy, guilt, the social contract, etc. At a deep level, it's subjective, but it's also so fundamental and intrinsically part of our culture and personalities that it may as well be an absolute.
There is no reason to not eat animals, because animals only have the value you assign to them. When I told him that humans assign value everyday, AKA why there is punishment for murdering a human, he replied that values and laws are assigned only for us to 'cover our own asses', and that there is no more value in a plant than in an animal. I was taken back, because I had never dealt with this philosophical approach before, and I am not sure what to think.
He seems to be walking a fine line between ethical subjectivism and sociopathy. Value and laws exist for a few reasons. One is definitely to cover our asses. We don't want to be killed, so we have to agree not to kill people. However, we also don't want our friends and loved ones to be killed. We feel real, sincere grief. We don't even want _anyone_ else to die, because we have empathy. And we don't want to have to kill anyone, because we feel guilt. Are those all subjective, and some even unique to certain cultures? Sure, but that doesn't make them less real. It doesn't make them less valid.
On the other hand, we have these values because we don't want to feel grief or guilt or sadness, so at their core they're all based on selfishness. Don't kid yourself that altruism really exists.
If someone doesn't feel that empathy or guilt or grief, it's no longer in their self-interest to avoid those actions. And those people are sociopaths. I really can't say which side your friend falls on.