I agree, having a crush or attraction to someone else is normal, it's something that happens when we are in relationships or not. Human nature, can't be controlled. But I am concerned that he told her about it. I feel like that is enough of "acting on it" to question his boundaries. To me, telling someone you are interested while in a relationship is similar to asking if that door is open. And she replied it is. If her answer is or becomes a simple yes, where does your fiance draw that line? I think couples counselling would help you talk through this and build good boundaries