Ruling of Niqab:
Niqab is a means of covering the face. Covering the face for women is permissible. However there is difference of opinion on whether it is obligatory. There are some madhabs who consider it obligatory. And every madhab considers it obligatory when there is risk of fitnah: such as when the woman is young and attractive and when society has extensive moral corruption.
ولا يبدين زينتهن إلا ما ظهر منها
And to not expose their adornment except that which [necessarily] appears thereof
— Quran 24:31
أي لا يظهرن شيئا من الزينة للأجانب إلا ما لا يمكن إخفاؤه، قال ابن مسعود: كالرداء والثياب،... وقال بقول ابن مسعود: الحسن، وابن سيرين، وأبو الجوزاء، وإبراهيم النخعي، وغيرهم
Means, they should not show anything of their adornment to non-Mahram men except for whatever it is impossible to hide. Ibn Mas'ud said: "Such as clothes and outer garments" ... Al-Hasan, Ibn Sirin, Abu Al-Jawza', Ibrahim An-Nakha'i and others also had the same view as Ibn Mas'ud
— Tafsir Ibn Kathir
يا أيها النبي قل لأزواجك وبناتك ونساء المؤمنين يدنين عليهن من جلابيبه
O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves [part] of their outer garments.
— Quran 33:59
هذه الآية دلالة على أن المرأة الشابة مأمورة بستر وجها عن الأجنبيين
This verse is evidence that a young woman is commanded to cover her face from strangers
— Tafsir al-Jassas
وتمنع المرأة الشابة من كشف الوجه بين الرجال لا لانه عورة بل لخوف الفتنة
A young woman will be stopped from uncovering her face in the presence of men, not because it is 'awrah but because of the fear of fitnah
— Durr al-Mukhtaar- (Hanafi)
Request of the husband:
Obeying a husband is obligatory for a wife.
الرجال قوامون على النساء ... فالصالحات قانتات ... فإن أطعنكم فلا تبغوا عليهن سبيلا
Men are in charge of women ... So righteous women are devoutly obedient ... But if they obey you [once more], seek no means against them.
— Quran 4:34
لو كنت آمرا أحدا أن يسجد لأحد لأمرت النساء أن يسجدن لأزواجهن لما جعل الله لهم عليهن من الحق
If I were to command anyone to make prostration before another I would command women to prostrate themselves before their husbands, because of the special right over them given to husbands by Allah.
No woman can fulfill her duty towards Allah until she fulfills her duty towards her husband. If he asks her (for intimacy) even if she is on her camel saddle, she should not refuse.
— Abu Dawud, Ibn Majah
The limits of this obedience are disagreed upon.
It is agreed upon that it applies at least to obedience in sexual intimacy.
إذا دعا الرجل امرأته إلى فراشه فأبت أن تجيء لعنتها الملائكة حتى تصبح
If a man Invites his wife to sleep with him and she refuses to come to him, then the angels send their curses on her till morning.
It is agreed upon that it applies at least to matters related to the above. Such as on obeying the husband when he commands her to not go out of their house or when he commands her to not let someone visit her in their house.
حقكم على نسائكم فلا يوطئن فرشكم من تكرهون ولا يأذن في بيوتكم من تكرهون
Your right over them is that they are not to allow anyone on your bedding whom you dislike, nor to permit anyone whom you dislike in your homes.
— Muslim, Jami at-Tirmidhi
مسائل الإجماع والاتفاق في باب ... وعلى أنه يجب على الزوجة طاعة زوجها وملازمة المسكن، وعلى أن له منعها من الخروج
The matters of consensus and agreement in this topic are ...
It is obligatory on the wife to obey her husband and to remain in her house, and it is the right of the husband to forbid her from going out
— Mizan al-Kubra
There is disagreement on whether it is also obligatory to obey him in everything else (as long as it is not haram, nor violates her own rights). Some scholars have stated that it applies categorically because of the categorical wording of some of the primary texts:
وحقه عليها أن تطيعه في كل مباح يأمرها به
His right over her is obedience to every permissible command
— Durr al-Mukhtaar
What to do if the husband tells his wife to observe niqab:
The answer IMO is that she should observe niqab, as there is a chance that it is obligatory on her anyways. Following this madhab is the more cautionary approach.
And if it is not a religious obligation then it is obligatory on her due to the obligation of obeying her husband. And various scholars give fatwa that a woman should observe face covering when the husband asks it. See islamweb and islamqa:
If she thinks that it is acceptable to uncover the face, he has the right to prevent her from showing it before non-mahrams, and he has the right to make her follow his opinion, which is that it is obligatory to cover the face – which is the more correct view – and she does not have the right to go against him.