Speaking from experience, I don't think you can really "scare" him into stopping. What you need to do [if you plan on sticking around with her that is] is get it through to him that no one is ever going to trust him.
You can emphasize the point by telling him "no" whenever he asks for anything that requires even the slightest bit of trust.
I remember when I was a kid, they always tried to scare us into not doing "evil things." To be honest, that just tempted me more.
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