Missvan - What exactly is the principle she is making? I didn't really get it from your post.

Is it that she holds the position that all carnivores are immoral? I surely doubt that as even the most extreme vegan does not hold that as a universal standard for all species.

I would imagine its the usual "I dont feel comfortable feeding rats to a snake" which is fine because she is not proposing a logical/moral claim but simply a matter of preference. You can just tell her "yeah and I like ice cream, so what?".

If she is saying "it is wrong to feed rats to a snake" then that is another matter, one which logic can tackle because she is making a universal claim.