Is the questioner considered a sinner for informing his colleague's wife of his sexual deviance, and is his marriage to her after her divorce considered a forbidden incitement (takhbib) according to Islamic law?

Informing the wife about her husband's immoral acts is permissible with the intention of preventing wrongdoing, but it is better to pursue other avenues, such as informing relatives. Interacting with a foreign woman [a woman who is not a mahram] through conversation without necessity is against Islamic law and opens a door to temptation. The agreement you made to marry each other is an act of incitement (takhbib) and a clear sin, and it is among the most egregious transgressions. Some scholars have even gone so far as to say that the marriage of a woman to the one who incited her against her husband is invalid.