Praise be to Allah the Lord of the Worlds.
Undoubtedly, forbidden relationships between men and women via the internet are amongst the great sins, which threaten families and society with moral corruption and instability.
Rather, these relationships are amongst the steps of the devil against which Allah the Almighty has warned us where He said," O ye who believe! follow not Satan’s footsteps: if any will follow the footsteps of Satan, he will (but) command what is shameful and wrong: and were it not for the grace and mercy of God on you, not one of you would ever have been pure: but God doth purify whom He pleases: and God is One Who hears and knows (all things)'{An-Nur, 21}.
However, we advise whoever discovered that his wife, daughter or son was having such relationship to resort to reason first and exercise patience until he/she adjust their behavior where he can use the "carrot and stick" approach to the necessary extent.
If he fails to rectify his wife`s behavior, then he is permitted to divorce her, but she gets the advanced and deferred portions of the dowry. In this case, he isn`t being unjust to her since there is a legal excuse for divorcing her, and that excuse clears him from liability before Almighty Allah. Not only that, but scholars have stated that it is desirable to divorce the "Unchaste" wife, as mentioned in the book "Tohfat al-Mohataj, 2/8" by Ibn Hajar Al-Haitami. And Allah the Almighty knows best.