Her husband is asking her to take off her hijab
Question

What should a wife do if her husband asks her to take off her hijab, knowing that she has tried hard to convince him that it is obligatory, and that there is no obedience to any created being if it involves disobedience towards the Creator?.
Praise be to Allah.If what is meant is that he is ordering her to uncover her hair and neck, etc, this is a command to disobey Allaah, and there is no obedience to any created being if it involves disobedience towards the Creator. There is no difference of opinion among the Muslim scholars that it is obligatory for a woman to cover her hair, neck and arms, and it is not permissible for her to take off her hijab in obedience to her husband or father, rather she should stand firm and seek help in that from righteous people in his family and hers, and those who have some influence on her husband. 
Allaah says (interpretation of the meaning): 
“It is not for a believer, man or woman, when Allaah and His Messenger have decreed a matter that they should have any option in their decision. And whoever disobeys Allaah and His Messenger, he has indeed strayed into a plain error”
[al-Ahzaab 33:36] 
“But no, by your Lord, they can have no Faith, until they make you (O Muhammad صلى الله عليه وسلم) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission”
[al-Nisa’ 4:65]
This husband has to submit to the command of Allaah, and accept His decree and not object to His commands. He should help his wife to follow the path of guidance and righteousness, and not prevent her from following the path of Allaah or call her to disobey Him. 
But if what is meant by taking off hijab as mentioned in the question is that he is telling her to uncover her face and hands only, and to cover everything else, and she is afraid that the matter may lead to divorce, and she will be harmed by divorce, then we hope that there will be no sin on her if she uncovers her face and hands, if she is forced to do so. 
And Allaah knows best.