To be certain, and as a courtesy to your friends, you should call Disney Dining and see if she can be added to any reservation. But the answer will most certainly be "no". (I am not certain that the person in Central Reservations has any way of knowing if a 9th person can be added to a table of 8. I think all they can do is see if there is availability for a new reservation of 9, and then cancel your 8. If that is true, the answer will almost always be "no".) At least this way you can explain to your friends that you tried. Then, when you are there, you can try to see if they can make an 8 round into a 9. You will probably have better luck at the restaurant because only the manager can truly know if they can fix the table for you. A manager will almost always say "I'll see what I can do", but a person on the phone will almost always say "It can't be done." If your friends know ahead of time that you tried and couldn't change the reservations, then if their daughter gets turned down at the door, at least they know that you did all you could. And if she gets in, they will think that you were sprayed with pixie dust.