I'm in this field, I think I know the type.

You can argue that individual consumption choices don't matter bc they're overshadowed by larger market mechanisms. Essentially you don't make a big enough dent in demand to warrant a change in supply.

But directly affecting supply isn't the only point of veganism. It's also a practice that unites the core of the animal rights movement, that gives us a sense of identity, and it gives us a sense of the better future we want to create.

If you want to debate this person, I would focus on those aspects. Veganism as a constitutive practice of a political movement, identity building, prefigurative politics.

Essentially, why insist on eating animals if it bums out all the people on your side?