Answer
If the girl is not a Ahlul-Kitaab (Christian or Jew), then it is not permissible to marry her. If she is a Christian or Jew, then it is permissible to marry her on condition he is firm on his faith of Islam. If he is weak and there is a fear of him losing his faith, then it is prohibited for him to marry a non-Muslim Christian or Jewish woman.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai