modification of custody for a runaway

My best friend had a baby when she was barely 15 years old. The child is now 15. "Mary" kept the little boy until he was 3 years old, but was too young to take care of the child and had little support from her parents. Child protective services stepped in(were called) and Mary's aunt agreed to raise the child so he could stay in the family until such time Mary could support him herself. The aunt got custody of the child (told Mary she adopted him...Mary was too young to know the legalities and her rights)and immediately refused Mary any contact with her son. Mary did not abuse or neglect the child but the aunt completely went back on her word. For 12 years the aunt has been telling the child "mom didn't want you, you should be happy to have someone to care for you" etc. All the while, she wasn't accepting phone calls, letters or gifts from Mary to the child. She threatened to call the police if Mary came to their house. This went on until a couple months ago. The child found out Mary's phone number and began calling her. The child was in a miserable situation with the aunt and uncle. He wanted to come to live with Mary who has wanted to be with him for years but thought, since he was adopted, she had no chance. To complicate things, the child ran away. Called Mary to come get him in another state and begged not her not to tell. Mary wanted to get him off the streets, so went to get him. Mary called the aunt and asked her "what now". The child said he will run away for good where "no one will find" him if he is forced to go back to the aunt. The aunt told Mary she can "have him." Mary plans to get an attorney to modify the custody order since she now knows there was no adoption. Would it do any good to get a notorized letter from the aunt stating that she will allow the child to stay with Mary, the biological mom, while the court proceedings are taking place and that she agrees to the modification? Will a document like this protect Mary from being arrested for harboring a runaway if the aunt changes her mind? They are so happy to be together again, the child left with the clothes on his back and doesn't even want the aunt to forward his things...he just wants to be with his mom. Any quick forms to file for temp custody since school starts soon? Mary lives in fl (where the custody order was filed), the child was living in GA.