What is the ruling on continuing a relationship and communication—via phone and messaging—with a married woman with whom the questioner had a relationship before her marriage, knowing that she is unhappy with her husband and wishes to separate from him?

You committed a sin by addressing this woman before her marriage and after it. The latter is more sinful because it involves corrupting a wife against her husband, and the one who does so is not of us. You must sever your relationship with her and repent to Allah by feeling remorse, abandoning the sin, and resolving not to return to it. Be sure to honor your parents and consult them regarding marriage. We ask Allah to turn your heart away from her. Treat this infatuation as Ibn al-Qayyim prescribed, and do not be deceived by her sweet words.