While it's fine to want to be alone sometimes, this girl may be in danger of becoming a social isolate & developing social phobia, which is not healthy for her mentally. She should seek some counselling.

i think all the advice above are useful. buy the way ,do not forget walking , esp in the street , or playgroud or by the river... alone . i promise,it's you won't feel bad.

Reading, watching movies, doing some cooking and contemplating. But if you don't know what you should do by yourself, maybe try to stay in touch with your friends will be better.

Take up a hobby. Listen to music, read, or even join a gym. I find music a comfort for me, whether listening and/or making music. I am not musically talented (wish I was), but you should try picking up a musical instrument and go with it.

thinking, becaus sure you are boring your time or you have just had a bad talk with someone; so think about the situation and the go with a freind to talk about it or just stay at home tohave fun with the members of your familly.