My niece found hidden cameras all over her house?

Reporting it to the police was definitely a good thing. If the husbands name is still on the house deed, he can basically do whatever he wants with the house. However, your niece could ask the court for a "cease and desist" order to make him stop snooping into your niece's privacy. You can ask the police station on how to do that and also ask them about other alternatives.