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 not from the People of the Book, then his marriage to her is void in principle and he is obliged to separate from her as she is forbidden for him. However, if she is a woman of the People of the Book, then his marriage to her is valid.
There is no doubt that abortion is forbidden as clarified in Fatwa 83677, and what we understood from the question is that abortion had already taken place and it is the one who did it who is sinful. It is not correct to say that her husband is an accomplice in the sin just because he travelled away without divorcing her. There is no doubt that he is obliged either to keep her in an acceptable manner or divorce her according to acceptable terms, so he should be advised about this. For more benefit, please refer to Fatwa 90663.
Finally, it should be noted that if a child is born on the bed of the husband then he should be traced back to him and he is not considered as an illegitimate child and the husband cannot refuse to attribute him to him except with Li’aan (i.e. he and his wife invoke the curse of Allaah on the liar among them when he accuses her of having committed adultery]).
For more benefit, please refer to Fatwa 89532.
Allaah Knows best.