Did the hostess serve an adequate meal? Yes? Then she was not rude. She's not obligated to serve a meat dish or any other signature dish. It's her choice to do so or not.
In the interests of full disclosure, I am a vegetarian. But what I find rude about the attitude that the hostess "deprived" the majority of her guests is that hints at entitlement on the part of any of the 7 who are offended. The first hint I got that someone felt they were entitled to or would feel deprived of my signature dish, would be the time I'd stop making it for parties altogether. We could insert any other dietary need/choice and I'd feel the same: low fat meal, low carb, gluten-free, low sugar or salt, etc.
And there are so many reasons that a person might chose to base the meal around one guest. It's a good way to get yourself to stretch your cooking boundaries when you otherwise might not: try new recipes and menu configurations. Maybe meat wasn't in the budget. Maybe she's trying out some dietary options of her own. Maybe she just isn't in the mood or it was a whim on her part. The assumption that she did it solely to indulge the vegetarian is pretty interesting and possibly not at all accurate. Even if she did, she would not be rude.