Her husband is asking her to take off her hijab

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.