This is something that I am struggling with also. I know that people are paying to attend, but besides the money issue I don't want random people I don't even know at our wedding. This is a special time in our lives for those who love and support us. I have asked my girlfriends who aren't married or in serious relationships to room with one another. So I can save room for family and those who don't have anyone to come with. They don't like it, but oh well!
But if she doesn't have anyone else to travel with, then I guess she has to bring someone. I don't know it's a tough situation. But like other ladies have said, at least she asked you first.