Answered by Shaykh Amjad Rasheed
What is the ruling of man who leaves his wife alone in a foreign country so that she can call people to the religion of Allah?
A woman living alone in a foreign country is not forbidden as long as her honor and religion is safe, whether she is there to call people to Allah, Most High, or for another purpose that is permissible, such as visiting relatives and the sort.
However, it is not permissible [in the Shafi`i school] for her to move from the country that she is in, such that she would be traveling alone without a mahram, even if it is with the intention of calling people to Allah, Most High. It is also impermissible for her to live alone in a place where her religion and honor is not safe.
Amjad Rasheed
Is there a dispensation in the Shafi`i school for a woman to travel with a safe group upon need when she does not have a mahram (unmarriageable kin) that she can travel with?
الجواب : إقامةُ المرأة وحدَها في دولة أجنبية غير ممنوع طالما كانت تأمن على عرضها ودينها سواء كانت تقيمُ هناك للدعوة إلى الله تعالى أم لغرض آخر مباح كزيارة رحم ونحو ذلك ، لكن لا يجوز لها أن تنتقل من بلدتها التي هي فيها فتسافر وحدها من غير محرم ولو كان ذلك بقصد الدعوة إلى الله تعالى . وكذلك يحرمُ إقامتها وحدَها إن كانت لا تأمن على عرضها ودينها .