I dunno what I would do in that situation. Can you talk to the kid? Maybe get her to open up if you feel comfortable in doing that? Even just knowing there's someone supportive around might help some. Is there an other adult in her life responsible for her that you could also talk to about your concerns?
I know if anyone asked me at that age, won my trust, and offered to just be a safe person to talk to, it would have made a world of difference.

It's a really tough spot to be in though, and I am not sure I would know totally how to approach any of what I suggested. I would be careful though, as you say you are still struggling with it. Just be aware of how much you are triggered by all this, and make sure to take care of yourself too