(And here's where the double-edged sword of custody comes in).
Here's what we have. A teen who for whatever reason not in the custody of her parent/s, but of a third party.
We have a Mom who is willing 'sign something' to help the teen get out of where she currently lives - but who won't report to child services that the teen might be at risk.
Now, Mom can try to obtain custody of the teen but here's the rub.
If the conditions are so bad at the grandparents, child services would have been involved.
And if they're not that bad? There's no reason to change custody.
It can't be played both ways - it's either bad enough to involve child services, or it's not.