My daughters mother sent her to live with her grandparents, i never signed my rights away. can i go get her?

i'm not sure of the legalities or how to go about this. but a few years ago my husbands ex was not doing so well and she sent their daughter to another state to live with her grandparents. he was not in a position to take her either. he is happily married now and has a beautiful home and 2 step daughters and a son of his own. we would like to have his daughter come live with us. she will be 13 this month. he never signed my rights away to her and he never will. he really wants his daughter back.. can he just go get her? he sends birthday cards and christmas and letters often. but no response. the mother doesnt have contact with her either. the grandparents wont allow it. when really they have no rights to his daughter at all..