When the battle with Lou starts at the end of the game on Expert mode, strum 
without holding any frets
until you lose. At the "Game Over" screen, select "Tutorial". Pick the 
tutorial for Guitar Battle, and play
through it until the "Lesson Complete" message appears. As soon as the next 
fret board appears, pause
the game, and exit to return to the lesson menu. Then, press Red to return 
to the list of songs. The battle
with Lou will be highlighted. Choose the song, and Lou's notechart will now 
be set to Easy mode.
Note: Your notechart will still be in Expert mode, but his will be in Easy 
mode