It's usually not too much trouble to divide portions of some dishes, making one of them meatless. My father used to make me special baked beans, cornbread dressing, hush puppies, beans, and lots of other stuff without onions, so I guess I'm used to this way of thinking. Don't assume that a vegetarian will eat eggs, fowl, fish, or even dairy. Your best bet is to ask people what they do eat. Vegetarianism means something different to everyone, and if you ask, your loved one will know you care enough to make sure that they enjoy the holiday, too.
Some people err on the side of being too cautious when it comes to preparing dishes for vegetarians.
For example, my husband doesn't eat eggs, but he does eat dairy, sugar, regular flour, etc. With this in mind, please try not to be judgemental of what people you love choose to eat. Many a holiday meal has been ruined unnecessarily by debating lifestyle choices. It means nothing about you if your family member or friend eats differently than you. If preparing something vegetarian is just beyond you, enlist the vegetarian guest to prepare something special and to make enough so that others can have a bite.