Boundaries of Empathy and Their Consequences

(Meta: It's cool that you changed the post in response to feedback!)

I wouldn't recommend trying to hard to shift the views of that single coworker, unless the two of you are close enough friends that you can keep pushing every so often without annoying her too much.

On your question (a), I think the answer is "yes", in the sense that some people will naturally see much greater morally relevant differences between humans and animals than other people. If you take your co-worker at her word, she may literally "not see it that way".

To be frank, I have a very difficult time empathizing with farmed animals, and accept arguments about suffering based on biological evidence (and trust in the rest of the community) rather than innate feelings. If I weren't surrounded by people and research pushing me to care, I don't know how I'd feel now. If pushed, I can articulate reasons why I struggle to empathize despite my rational knowledge, but the reasons will sound silly, and I have to dig to get past my natural apathy and actually find the reasons. Your friend may never have done that kind of digging.

On (b), I very much doubt there are generalizable answers. For every major film about animal welfare, there exist people whose views were transformed by the film and people who watched it and didn't change their views at all. The same is true for every relevant book, every veg*n argument, and so on. People are different along so many different axes that you rarely find a "general" path to persuasion, especially on a cause that entails such a different way of thinking about the world (and acting in the world).

This doesn't mean you have to give up on persuasion, though. My response to developing this view was to take causes I cared about (e.g. the general case for effective altruism) and develop a collection of distinct arguments/frames, so that I could try to shape my persuasion to match any person I spoke with. The same approach might work well for animal advocacy; there are arguments you could use to appeal specifically to libertarians, socialists, pet owners, environmentalists, Christians, and any number of other groups.

Yes! It's much more conducive to conversation now, and I've changed my vote accordingly.

To actually engage with your question: I personally find (1) to be the most motivating reason to adopt a more vegetarian diet since I'm more compelled by the idea that my actions might be harming other persons. Regardless, (1) and (2) are both grounded in the empirical observations. (and both of which are seriously questionable in how much of a difference they make in the individual case: see this and the number of confounding factors in veg diets causing better health)

I personally reject (3) because animals don't fall, in my ontology, under the category of morally significant beings (neither argument nor experience has yet made me think animals possess whatever it is that makes us consider, at least most, humans as persons) I take this to be a morally relevant difference. (Though, I would endorse many efforts to improve animal welfare for reasons ultimately grounded in human person welfare.)

Moreover, regarding changing behavior, I can think of a number of additional reasons someone might not change their behavior that aren't related to empathy, e.g. they might find it supererogatory, they might have ingrained cultural reasons, they might not think they'll be able to make a difference, and reasons to do with poverty and food injustice.

Thus for me, an answer to (a) and (b) would be a convincing theory of personhood and a further convincing argument that animals share that person-making feature (or other moral relevance-making feature).