A simple answer would be to call her and tell her "all the seats are now taken by paying students, so there will not be room for spectators, and you knew she would understand, but you can't wait to see her in another advanced class." Gets the message across without any uncomfortable confrontation. (If the seats are not all full at class, you can always remark something about inconsiderate people not calling you, and move on.) Everybody stays happy.
You could also ask her to bring her friend by a day or two early so she will know you at least. I love it when people offer this because I am shy too!
Last edited by madamekelly; 04-12-2015 at 10:44 AM.