And the fact that she not only met him, but actually DEFEATED him would probably send her into some form of hysteria.
And my idea for when Star Swirl was actually around maybe the age of Twilight's parents when he had Clover the Clever as his student, and when Celestia and Luna came along, Celestia made him her student, which eventually led to possibly Star Swirl trying to cast the spell the Elements used to make Twilight ascend to an alicorn, but because he didn't understand friendship, he became Discord instead. Or maybe he tried to use the Element of Magic. I mean, did we all see what Sunset Shimmer looked like? I think that that would've been powerful enough to do something like that to Star Swirl.