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.
If this woman is a non-Muslim and she is not from the People of the Book, then his marriage to her is void according to the consensus of the scholars . In which case, he is obliged to separate from her but if she is a non-Muslim but she is from the People of the Book [i.e. either a Christian or a Jew], and she is chaste, and he married her with the consent of her guardian, then his marriage to her is valid; for more benefit, please refer to Fatwa 84456.
However, he is obliged to be just between you and her but it is not permissible for you to ask him to divorce her.
As regards what you mentioned about him that he wrote and signed a divorce paper, then if a husband writes or signs the divorce paper, then this is a metaphor of divorce as clarified in Fatwa 118112. In which case, if the husband intended divorce, then divorce takes place whether or not the wife signed this divorce paper because in Islam divorce is in the hands of the husband as we clarified in Fatwa 90056.
What you can do now is to contact this man and ask him, or that your guardian contacts him in order to find out from him about the matter of the divorce which he wrote. Therefore, both of you should either reconcile between yourselves or separate in kindness. However, if you do not hear about him, then you should take the matter to one of the Islamic centres who follow the methodology of Ahlus-Sunnah wal-Jamaa’ah so that they would study the matter and repel the harm off you [i.e. by granting you divorce].
Allaah Knows best.