My exwife, when I admitted that I was an atheist, once asked me "If you don't believe in god, what's to stop you from going out and raping and murdering?"
Does your ex believe what the bible says? Have her read this at Romans 2: 14, 15;
14 For when people of the nations, who do not have law, do by nature the things of the law, these people, although not having law, are a law to themselves. 15 They are the very ones who demonstrate the matter of the law to be written in their hearts, while their conscience is bearing witness with them, and by their own thoughts they are being accused or even excused.