Should I uninvite a dinner guest? Can I?

That is extremely rude and ungrateful of your friend. Simply tell her that you would love to have her but you have already finished your food shopping. You have a menu set and really are in no position to go out of your way to add additional items to the menu. If you really want to uninvite her, you could always calmly and politely explain to her that relatives invited themselves last minute and their tastes will most definitely conflict with hers. (After all, why make vegetarian food if only one person will be eating it?). I understand she's going through a rough time, but that is no excuse for taking advantage of one's friends, especially around the holidays. If she persists in being annoying about the menu ask her to bring a dish for everyone to enjoy and explain that there are items that you're sure she would enjoy if she tried them. I understand your frustration; I shudder when I have to cook for vegans/vegetarians and to be a picky eater on top of that is just too much to impose on someone. Best of luck and don't let her ruin your holidays.