(Insert 2 hour Lecture here)
Burning sage is a really good thing to do. If this doesn't work however, you should consider talking to her, and from there, once you have figured out what the child wants, you can have your friend light a black candle, and command the spirit to exit the body. Touching a bit of cinnamon or a piece of copal to the forehead of her is a good way to force a spirit who doesn't want to leave out.
Be warned now though, that since your friend is very open to this now, she can very easily have this happen to her again. I would advise that you take more precautions next time you want to play with something you don't understand....like not doing it to begin with. If you have questions on these things, teachers on this site are here for a reason.