Frogman, Improvise means to create and perform spontaneously or without preparation. Improvise comes from the Latin word improvisus, meaning "unforeseen, unexpected." Think about when something unexpected happens to you — you have no choice but to react in the moment, or improvise.
I'm sure he, (meaning my musician friend), practiced so much that he could visualize his instrument in his sleep during his formative years. Why do so many musicians play with their eyes closed? That's because they don't need to even see the thing their playing, even when it's as complex as a "Koto". I saw June Kuramoto play the Koto most of the night with her eyes closed; she was playing improvisational jazz better than on any records I have of "Hiroshima"; with her eyes closed, I even have pictures.
You see classical musicians practicing morning noon and night; I'm sure it must be necessary; but if you are going to be improvising, playing new music that you have in your head, what are you going to practice? When improvising, each note you play, depends on the note someone else played before you; that's why good jazz musicians sound so good, because if you play a wrong note, someone else can play a note that will make it sound good, and it all comes out sounding beautiful.
Although you keep harping on the "fact" that my friend did not practice when he was performing at least three nights a week; the elephant here is "improvisation"; that's the ability to create "spontaneously"; either you got or you don't, but no amount of practice will enable you to do this.
Enjoy the music.