DIVORCE AND PASSPORT OF A CHILD

My niece who lives on Dallas, has two kids. Her husband will not allow my niece to make a passport for her 6 month old daughter, the other daighter has her own passport. I understand that the application for a passport requires both parents' signature and he has flatly refused to sign the application and he will not give her any reason for this. He has mentioned divorce to her but so far he has not taken any action. Both of them are not getting along due to problems related to husband's parents who are living with them and making things difficult for my niece. What should she do on this case scenario?