Answer:

 

And may peace be upon you!

 

This humble servant is not at all worthy of the valuable titles you have given.

 

In the case of the question, Zayd’s wife is not out of marriage. Because no legal aspects of breaking the marriage are perceived. Nevertheless, if he accidentally had sex with the mother, the mother will then be haram to Zayd’s father. Because in Islam incest [hurmat musahara] becomes evident in having sexual intercourse with the wife of the father. And she will be haram to the husband forever, that is the father of the one who had sexual intercourse. The relatives for whom incest becomes apparent, among them are also the father’s wives. If sexual intercourse has occurred accidentally, even then incest becomes apparent.

 

It is stated in Fatawa al-Alamgiri:

The father and grandfather’s wives are haram forever, just as it is stated in al-Hawi al-Qudsi. And with them, a sound marriage becomes incest, not with an unsound [fasid] marriage, as it says in Muhit Sarkhasi. And incest becomes apparent through halal sex, or by mistake, or by fornicating [zina] with them, just as it stated in Fatawa Qadi Khan. Likewise, in al-Dhikr there is the condition of lust in the sense that if a four-year-old child had sex with his father’s wife, incest does not become apparent, as it is stated in Fath al-Qadir.

[Fatawa al-Alamgiri: vol. 1, p. 274-275 – الباب فی بیان المحرمات]

 

And Allah Almighty knows best with rectitude.