What is the ruling on a suitor's attempt to dissolve the engagement of a woman already engaged to him for the purpose of marrying her, knowing her desire for him, and should the fiancé be informed of this matter?

You must sever your relationship with the girl to whom you are bound [by marriage contract to another man], because enticing her to separate from her husband is considered takhbib, which is forbidden in Islamic law. It is one of the great sins and an act of sorcerers and devils. You must repent to Allah and abstain from the unlawful by enjoying what is lawful. If you need another wife, then pursue lawful means, while observing the condition of justice.