Yeah that was a wrong decision on the vegan’s part. Perhaps this sort of behavior might be acceptable in the public commons, but work is a private space where people join a company for specific purposes. Work and philosophy/politics should not intertwine.
And who knows: if she excluded herself from the breakroom during lunch without notifying others, maybe coworkers would notice and be more willing to hear her out out of a desire to socialize. It probably could have helped her effort to do this actually.
Vegans live and learn. We are part of a minority group, and with being a minority comes all of its benefits and detriments. We just need to learn that in situations like these, we often are the only vegan around people and so we need to carry our entire movement on our shoulders, whether we want to or not. Else, you get general, anecdotal sentiments the likes of which you see in this post.
Unfortunately for any minority group that seeks change within a group led by the majority, this is true. Perhaps the vitriol against vegans is part of the game of realizing change: there will always be resistance and tendency from some portion of the population to keep things the same as they always were, regardless of whether those things are good for the population itself.