So you know for certain that this person is a 13 year old? That you've known her through real life means and there can be no doubt she's who she says she is, because for local authority to step in or CPS or any other agency, they would need her real name and pure facts. And if she is a preteen in trouble, her parent(s)/guardian would need/should get involved. Do her parents play too? If no, than who paying for her subscription each month?

SE has the right of it, they can't take hearsay from outsiders or they could get lawsuits all the time. Does the person who is supposedly manipulating her, know how old she is? These questions I would be asking of her.

I understand you feel obligated to report and you say you've done so, but that's about all you can do on your end. The rest is up to her. Maybe she'll come around and file a report, but I highly doubt it. She knows the aftermath of doing so and what it will bring.