I've definitely worn leather/suede to vegan restaurants before. It never even occurred to me not to, or that it would offend anyone, and I pretty much always have leather on me somewhere.
I've never thought of my clothing as being related to what I was eating, unless the restaurant had a specific dress code.
I don't see her as rude unless the restaurant itself has a no-animal-products dress code. I like eating vegan sometimes but that has nothing to do with what I'm wearing. Even if I ate vegan all the time, it could be for health reasons, or because I disagree with practices done by the meat industry, and not because I disagree with any use of animal products including clothing.