I don't think hosts have an obligation to alter a menu planned in advance for a last-minute guest with dietary restrictions, but if I were in your shoes I'd probably try to do something. For example, you might look up some vegan Caesar-type dressing recipes and have a bowl of that salad handy. Make more, as it's the only thing she'll be eating.
The vegans I know are used to having scant offerings at dinner parties and eating lots of salad, and since her invitation is last minute, she should understand. If she doesn't, she'll probably hate you and refuse to attend future dinners you host, solving future menu problems for you. Offer her a friendly apology that you don't have many items for her to eat, but you'll try next time to have a bit more.
If I were in your friend's shoes last week, I'd have asked what the menu was, then not invited her when I knew there were no dishes a vegan would eat (though maybe he didn't know that Caesar dressing has meat in it).
edited to add: And you need to put deadlines on the "invite a friend" permission to prevent future surprises. And Buffy, I hate grapes. They freak me out.