All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
First of all, you have done well by inviting this woman to Islam and encouraging her to embrace it; so may Allah reward you generously and make this endeavor in the balance of your good deeds.
We also ask Him to enable your friend to do good and make her heart firm on it. We advise you to supplicate Allah as much as you can to guide her, because if Allah knows that there is any goodness in her heart, Allah will enable her to achieve it. You should also remind her to supplicate Allah herself because if she supplicates with sincerity, then we hope that her supplications will be answered by Him.
Secondly, you should clarify to her that her embracing Islam and being steadfast on it until death means eternal happiness in Paradise, and that dying as a disbeliever [non-Muslim] means eternal misery in Hellfire. This eternal happiness outweighs everything from the pleasures of this worldly life and if a conflict happens where one either achieves this eternal bliss or obtain this worldly life's pleasure, then a wise person would certainly choose the eternal life over this temporary one.
Read to her some of the biographies of prophets and righteous people and how they were harmed and tortured, but this did not make them turn away or renounce their religion; rather, they persevered for the Sake of Allah.
Thirdly, clarify to her that if she embraces Islam and her husband remains as a disbeliever, then there is no good in him for her. Allah Almighty will compensate her with someone who is better than him.
Moreover, her husband has no right to deprive her of her children, and if he tries to do so, then she should take the case to the court (or to the authorities).
Allah knows best.