Is a husband's complicity, if he is aware of his wife's possession of nude photos of women on her phone, considered a falsehood, and what should be done about it?

Your husband committed a grave sin by photographing women and keeping their pictures, and the sin is even greater if they were naked. He must repent and destroy the photos. If you can destroy them without harm, then do so. Otherwise, advise him and warn him of Allah's punishment. If he responds, praise Allah. If he does not, there is no sin upon you, for you have fulfilled your duty of giving advice.