Honestly, i would have told him to f*** off! What a prick!
I got up at a jam night once and jammed Sweet Child o mine. It was the first tune I'd learned to play like 20 yrs earlier. I was just jamming on it, put some of my own stuff in but nothing detracting from the line. A guy come up to me and said "Quite good that, I'll show the exact part if you want"
I told him to f*** off too!