Grandma would like to relinquish custody of granddaughter to the bio mom… how would that work?

Does anyone know what this would look like?

My mother took my niece in about 5 years ago. She is now 15 and totally defiant and claims she wants to go back to her bio mom.

Her bio mom relinquished custody of her 5 years ago at 10 years old due to financial issues and instability (supposedly), and my niece went straight into foster care. My mom found out and quickly adopted her legally. But now, my niece is really causing my mom such a headache, my mom is at an old age and my niece has become just uncontrollable.

My niece claims that she just wants to go back to her bio mom… her bio mom now lives in her friend’s basement rental, has a minimum wage job, and her bf has a long criminal history of Misdemeanors (some as recent as literally last week).

My question is… can the grandma give custody BACK to the bio mom? Even if the bio mom makes minimum wage, lives in a basement, and has a bf who has a recent criminal charge? This would be beneficial for my niece’s bio mom too because she would no longer have to pay my mom child support.

We really want to avoid my niece going back to foster care. The bio mom has claimed she open to taking back custody of my niece.

My brother (niece’s father) is not in the picture whatsoever due to complicated situations.