My daughter gave up custody of my g-son to me and the father now she is commig to get him what do i do?

my daughter gave up custody to go into the military i have half and the father has the other. i have power of attorney. i have no proof of the custody and the father that does is out of town. she is comming any day now that he is out of town to take him. she is mentally ill and is pregant with a granddaughter she is giving up possibly sell her. i need to know what to do