First off, you can't really cheer her up until you ask her what's wrong, or tell her the truth about what you heard.
When she knows you know, maybe try to tell her this stuff happens and it's not her fault and stay positive. Give her some long speech kind of thing, then try not to mention it as much because it may bug her.
Also, give her the number to the Kids Help Phone, it might make her feel better to talk to someone she doesn't know and get some help. I don't know it but look online for it. Hope all goes well.
