Early in the script, Ferris goes on a spiel about a kid he used to be friends with in the eighth grade, Garth Volbeck:
Ferris explains that Garth came from a troubled family and even had a psycho older brother who once ate “a whole bowl of artificial fruit just so he could see what it was like to have his stomach pumped.” Ferris was apparently the only person who ever tried to help Garth and be a friend to him, but Garth would eventually drop out of high school and get into even more trouble.
The scene with Charlie Sheen goes pretty much exactly like in the movie, except for this part:
This backstory actually explains Ferris’ whole character: Ferris claims that his entire motivation for skipping school that day is to help his troubled friend, Cameron, but we always assumed that it was just an excuse to take a joy ride in his dad’s Ferrari. However, the fact that Ferris had previously tried to help another friend and failed miserably makes him seem a lot more sincere about wanting to help Cameron. Garth represents Cameron’s possible future if Ferris doesn’t do something.