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 is His slave and Messenger.
First of all, all perfect praise be to Allaah Who guided your friend to Islam and we ask Him to make us firm on Islam until the Day we meet him.
Undoubtedly, not wearing Hijab is a grave sin as she is obliged to wear it, and it is not permissible for her to obey her husband in disobeying Allaah. Whoever seeks people's contentment by angering Allaah, Allaah will forsake him and will make people displeased with him.
Hence, she should remind her husband with Allaah and with His severe Punishment. Moreover, it is an obligation on him to be grateful that his wife became a Muslim and is steadfast on the obedience of Allaah; indeed it is a great bounty from Allaah. So he should help her in obeying Allaah and not stand in her way as an obstacle.
If her husband does not accept her advice, then a trustworthy brother in your country should advise him.
For more benefit on a woman whose husband prevents her from wearing Hijab, please refer to Fatwa 96283.