All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If a woman refuses or procrastinates on wearing the Hijab, then she does not fulfill all her religious obligations. In fact, she has neglected a great obligation whose the non-fulfillment is among the major sins because the Hijab is an obligation that is imposed on the Muslim woman. Allaah Almighty says (what means):
• {And do not display yourselves as [was] the display of the former times of ignorance.} [Quran 33: 33]
• {O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves [part] of their outer garments. That is more suitable that they will be known and not be abused. And ever is Allaah Forgiving and Merciful.} [Quran 59: 33]
• {And tell the believing women to lower their gaze and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their head covers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allaah in repentance, all of you, O believers, that you might succeed.} [Quran 31: 24]
Those whom Allaah Almighty has mentioned in the verse are those whom a woman is allowed to expose her beauty in front of, while she is not allowed to do so in front of anyone other than them.
Your wife has no choice but to acknowledge the obligation of wearing the Hijab and hasten to wear it; Allaah Almighty says (what means): {It is not for a believing man or a believing woman, when Allaah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair. And whoever disobeys Allaah and His Messenger has certainly strayed into clear error.} [Quran 36: 33]
It is incumbent on you to explain the matter to her and advise her in a way that is best. If she refuses, then you have to order her to do it, and if she refuses and insists on her stance, then divorce her because leaving her in her current condition is a form of cuckoldry. ‘Abdullaah ibn ‘Umar narrated that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: "There are three categories of people whom Allaah will not look at on the Day of Judgment: one who is undutiful to his parents, a woman who imitates men, and a cuckold (dayyooth)." [An-Nasaa’i and Ahmad]
Allaah Knows best.