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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the matter is as mentioned that this woman has established an affectionate relationship with a marriageable man then she is indeed wrong and she committed a grave sin, especially that she is married. It was an obligation on her to protect herself during the absence of her husband. Allaah Says (what means): {So righteous women are devoutly obedient, guarding in [the husband’s] absence what Allaah orders them to guard. (i.e. their chastity, their husband's property).}[Quran 4:34]. If she persists in doing so, she is a disobedient woman and her husband should follow the legal steps in treating a disobedient wife which are mentioned in Fatwa 89480. Then, if she repents and becomes righteous, all the best, otherwise it would be better for him to divorce her as she might give birth to children who are not from him, and if her husband accepts her as she is, then he is somewhat a cuckold. But it is not permissible to accuse her of adultery without evidence.
Finally, the two following matters should be noted:
1- The husband should endeavour to provide the means for his wife to preserve her chastity, like facilitating her to reside with him according to his ability, or that he should return to his country to reside with her, or not be absent from her more than six months. In this regard please refer to Fatwa 85711.
2- It is not permissible to tolerate marriageable men entering into women without observing the religious requirements, as this causes temptation; in this regard please refer to Fatwa 89331.
Allaah Knows best.