Observance of hijab is obligatory according to the Shari'a, and according to a noble verse of the Qur'an in Surah Tahrir, which says (or, O people, protect yourself and my family from the fire of Hell); It commands them to do good deeds and religious duties and prevents them from doing bad and forbidden deeds, and observing the hijab is one of the most important things. It is obligatory on women in these matters, especially hijab and chastity, in addition to being an obligatory and necessary matter of religion, it is obligatory on them to observe it like prayer. Those who follow the religious precepts are saved, and those who do not wear the hijab or do not wear the hijab will receive nothing but torment.
The issue of covering has nothing to do with the order or prohibition of the husband, but it is the rule of God, and women should observe it in places where the hijab is obligatory, that is, in the eyes of non-mahrams, even if the husband objects.
You can strengthen their faith by giving information over time, and by creating fear of God and the Resurrection and encouraging by mentioning some verses and hadiths or through books in these fields, you can strengthen the desire to observe the hijab. Create them.
You can also acquaint him with the worldly and otherworldly consequences of leaving the hijab mentioned in the narrations.
Pilgrimages and understanding of spiritual spaces can play an effective role in encouraging them to wear hijab. So try to encourage them to travel like this.