How to deal with crybaby nine-year-old?

Chances are that someone is giving her attention for that type of behavior . You cant change what her parents do but you can make it clear to her that if she wants your attention that shes going to have to act rationally . If she whines or cries for no good reason , ignore her , don't speak to her or give in to shut her up , just act like shes not there. Eventually she'll ask why u aren't listening to her, take that time to tell her that u don't appreciate that type of behavior and if she wants the fun to continue that she will stop the incessant crying. She will learn that the way to get your attention is not by crying and whining but by speaking in a civilized manor. Have patience though it may take a few days for her to get the point.