How can one deal with and respond to a Muslim colleague, by name, who does not believe in the existence of God or in religious rituals, and who asks why God acquitted the wife of Imam Ali of the accusation of adultery and why the verses of acquittal were delayed in their revelation?

Denying the existence of Allah is an act of disbelief (kufr), and it is not permissible for a Muslim to associate with such a person. Rather, one must strive to guide them and engage them in dialogue with wisdom, or seek assistance through other means, such as bringing an influential friend, or providing beneficial books, tapes, and websites. There is no record that Ali had a wife whose honor was slandered.