I heard a quote, there is nothing being done that has not been done before. When I apply that principle to music it is also true. We will never play a note that has not been played even in the fashion and series in which it has been played.
What I find so fascinating about the creative process is how we try to put something together that does not sound like something already out there. Every artist will speak of those who influenced them. And if you have been exposed to that artist referenced you can begin to see traces of their influence within the artist being questioned.
To answer your last question, in my life it was seeing guitarists like Al Dimeola, Carlos Santana, Larry Carlton, Stanley Jordan and what they did with the instrument that did two things to me: one, caused me to hang out with the guitar because I had to know how they did what they did. Two, develop an appreciation for what they did because no matter what I did in those days I could not touch what I had heard. From there, it became an endless cycle from appreciation bringing about practice and back around again. It still happens to this day. What drives me to practice is the excellence of another. That is why I am kind of confused as to your friend not liking to play with other musicians. When you think of how iron is sharpened it is by rubbing against another piece of iron, I do not have any idea how your friend improves in his progress.
Cheers,
RickeG