Please just make sure the best that you can that the girls invited are liked among each other. I had a birthday party sleep-over in 6th grade and one girl I invited turned out to be unpopular with a handful of the other girls. She was treated very badly all night. My mother has a nervous, avoidant type of personality so we were not monitored at all, so that extra freedom really exacerbated the whole thing. I was so ashamed because I fell into peer pressure and played along a little even though she was my friend. Afterward the other girls told me I could not be friends with that girl anymore or they would not be friends with me. :|

It could be a really fun experience for her if it is kept structured, friendly, and safe for all girls.