All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is strictly forbidden to propose to a woman whose case of divorce is still under legal procedures, since she is still a wife and doing so is a major sin. The Prophet said: “The one who turns a woman against her husband is not one of us.” [Abu Daawood and Al-Albaani regarded it as sound]
Also, if some sensible, free man divorces his wife and the issue is presented to court for legal proceedings, it is forbidden to propose to her by any means until her 'Iddah (waiting period) comes to an end as long as the divorce is revocable. But, if the divorce is final, it is permitted for one to remark to such a woman by saying some indirect reference as: “I intend to marry, I pray to Allah to grant me a righteous wife.” This is the opinion of the majority of the Muslim scholars.
For more benefit on the prohibition of turning a wife against her husband, please refer to Fataawa 92056 and 420595.
Allah knows best.