I find that doing anything behind someone's back is a bad idea. Not like there is many options here, she either leaves him and continues the relationship with you or she leaves you eventually and lies to him by continuing their relationship. Maybe it all somehow blows up in her face and she is left with no one, but considering you are a fool for this woman I think you would be with her even if she was the village idiot hooker.

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Always and never are two words you should always remember never to use.