If his guitar amp has a "Line Out" or "Preamp Out", he can route that signal through a Y-cable with RCA phono plugs, and go into an auxillary, tape in, or other line level input on the stereo system. The Y-cable will allow the signal to go to both Left and Right channels.He may want to use a mixer of some sort, to first mix his guitar signal with a prerecorded backup, such as the Jamie Aebersold CD's sold by www.jazzbooks.com. Then from the mixer into a line level input on the stereo.