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.
It is not an obligation on the husband to inform the woman whom he wants to marry about his other wife, but if he informs her, then it is better. For more benefit, please refer to Fatwa 86395.
Therefore, you are not considered as cheating your wife [second wife] by this marriage, and your marriage with her is valid since she agreed to the marriage contract. However, if after that, she dislikes staying with you, then she has the right to ask for Khul’ in the same manner that the wife of Thaabit Ibn Qays did with her husband. For more benefit on Khul’, please refer to Fatwa 89039.
If you want to divorce her initially without any compensation, you may do so.
The following matters should be mentioned:
1- The guardian has no right to oblige the girl under his guardianship to marry someone whom she does not like.
2- The fact that there was no sexual intercourse between the spouses does not affect the validity of the marriage as we clarified in Fatwa 94224.
3- The husband must protect his wife from the reasons of corruption and prevent her from any sinful relationship with a non-Mahram man. Indeed, it is an evil act that your wife asks you for divorce because she wants to marry another man and it is forbidden for her to let herself be attached to him. It is also strange that you are not jealous about this matter, so you did not prevent her from this sinful relationship nor did you divorce her although she has already expressed her desire for a divorce.
Allaah Knows best.