The NY Times Motherlode column here has an interesting question. One of the readers has an eleven-year-child who does not want to go on a mandatory school overnight trip. There are many good suggestions in the comments that follow about what to do.
No, I certainly would not force a child. What might help is this. Give your child a cell phone—perhaps an inexpensive pre-paid one. Show her how to call you on it. Tell her that you will have your cell phone with you all the time. If she decides on the sleep-over that she wants to come home, she can call you and you will come and get her, no matter what the time is.
My mother was phobic, and taught me the concept of having an escape. Often, just knowing that you can leave enables you to stay.