I do have a problem with a 9yr old waiting in the main area of a ladies bathroom. The doors in most places have huge cracks, and it is very easy to see what is going on, if someone is curious. And I would think a 9yr old boy is most likely curious. If you don't want to leave your child outside while you use the bathroom, fine. But then either put him in a stall with the door shut, or take him in the stall with you and have him face the door. Or make him stand outside of your door so that his feet are underneath it. You have a right to do what you need to do to keep your child safe, I understand that. But you also need to be sensative to the other people who are using the facilities. I don't want anyones child peeping at me, and by taking any of the simple steps above we would both be happy.
Op--Is your friend afraid that something may happen inside the rest room, or that her child would get lost when he came out? I would think that with such a large group of people it would be easy for someone to wait outside for him. And even shout in for him to make sure that everything was going ok.