Anyway, can anyone shed light on what a normal emotional response is to a long-time friend leaving?
i think what you are describing is a normal emotional response. it is normal to miss a loved one who is moving away. its normal to maybe even feel some anger about that.
that is, of course, separate from how one outwardly responds.
"Hey, congratulations on your new venture! That sounds really exciting. We'll miss you around here but it sounds like you're on to something great so we wish you all the best."
this is a normal/standard outward response. its whats appropriate in a public/professional/business like setting. congratulations, well wishes, take care, youll be missed. one can personalize it of course, maybe throw in a favorite memory or two, or "im really going to miss you, friend", but "this really hurts and upsets me" wouldnt be appropriate in that kind of setting.
going beyond that entirely depends on the personal dynamic between you and the person moving.