Most vegetarians that I encounter I actually initiate an inquiry when it comes up; they aren't very pushy.
The pushy ones are the ones that tend to get noticed. Just like fundamentalists of any group.
To her, it was disgusting. To some people, seeing a dog roasted alive (or dead, even) is disgusting. "Disgust" is all about personal perception; and if she was not informed that a pig was going to be roasted, then I can see why she was appalled. Once more, it seems like your main concern is that she isn't MAN enough (er, woman enough?) to see a carcass and think, "Oh, hey, it's just dead meat that I can eat", and instead saw what was once a live animal.
Some people don't actually like seeing the whole of dead animals. Personally, I loathe being able to see the face of the animal that I'm eating. It's disturbing.
But then, I'm not a REAL man, I guess.
Okay, seriously, I find it interesting that you are so upset over her reaction. She was not informed that a whole pig was going to be cooked, right? And when she saw it, she was disturbed, and reacted in that sense. I don't quite see why her reaction was that bad; if she had never been exposed to a dead animal, I don't think she would have had the desensitization that, say, a redneck would have while he's tearing out the guts of a deer carcass.
ETA: I wouldn't be surprised if there was something else about this scenario that wasn't stated or that I'm just not imagining. When you mentioned that she was "preaching" how wrong it was, I can see that there's a possibility that it was much worse than I'm imagining currently...