Praise be to Allah.
We ask Allaah to grant you relief from your distress and to increase your faith and steadfastness.
What you have mentioned about your husband’s actions is something reprehensible with which Allaah and His Messenger (peace and blessings of Allaah be upon him) are not pleased, and of which His righteous slaves do not approve.
Romantic relationships between men and women are not allowed, and it is clearly haraam, whether that is via the internet or over the phone, or otherwise. If it goes beyond that to meetings and immoral actions, then this is the essence of doom.
Were it not for this ecstasy that your husband is experiencing, he would feel pain, alienation and confusion, which is what is usually felt by the sinner who persists in his sin.
Do not believe that he is living a life of pleasure and enjoyment, rather it is intoxication, negligence and a drifting away from Allaah, as Allaah says concerning those who commit immoral actions (interpretation of the meaning):
“Verily, by your life (O Muhammad), in their wild intoxication, they were wandering blindly”
One of the most abhorrent of actions is when a person commits his sin openly and boasts about it, not caring about the punishment that awaits him. Hence the Prophet (peace and blessings of Allaah be upon him) said: “All of my ummah may be forgiven except those who commit sin openly, and part of committing sin openly is when a man does something at night, then in the morning, when Allaah has concealed him, he says, ‘O So and so, I did such and such last night,’ when his Lord had concealed him all night but he discloses that which Allaah had concealed.” Narrated by al-Bukhaari, 6069.
You should praise Allaah for protecting you and sapring you, and making you better than women of that ilk. The Prophet (peace and blessings of Allaah be upon him) said: “Whoever sees a person who is afflicted with some trial and says, ‘Al-hamdu Lillaah alladhi ‘aafani mimma abtalaaka bihi wa faddalani ‘ala katheer mimman khalaqa tafdeelan (Praise be to Allaah who has spared me that with which He has tested you and has favoured me over many of those whom He has created),’ that trial will not befall him.” Narrated by al-Tirmidhi (3432) and Ibn Maajah (3892); classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
You should note that Allaah gives the evildoer respite until, when He seizes him, He does not let him go, as the Prophet (peace and blessings of Allaah be upon him) said: “Allaah gives respite to the wrongdoer until, when He seizes him, He does not let him go.” Then he recited (interpretation of the meaning):
“Such is the Seizure of your Lord when He seizes the (population of) towns while they are doing wrong. Verily, His Seizure is painful (and) severe”
Narrated by al-Bukhaari, 4409; Muslim, 2583.
So you should not be deceived by the fact that this woman continues to do wrong and seems to get away scot-free, for there is no barrier between the prayer of the oppressed and Allaah.
Perhaps you can find some good people who will advise him and remind him of Allaah, even if that is via a Friday khutbah, for example, condemning such haraam relationships and describing the punishments in this world and the Hereafter for those who do such things.
Make a lot of du’aa’ to Allaah, especially at times when prayers are answered, such as the last third of the night, between the adhaan and iqaamah, between ‘Asr and Maghrib on Friday. There is nothing wrong with your making du’aa’ against her because she is an evildoer, but it is better to pray that Allaah will improve your situation.
You have to be kind to your husband and make yourself beautiful for him, for perhaps that woman has captured his heart with kind words that he did not hear from you, or by making herself beautiful when you did not. So try to win him over in that way. And you have to be patient, for this is a test from Allaah by means of which your sins will be expiated and you will be raised in status.
And Allaah knows best.