Mr. Booker was a patient at Sacred Heart Hospital.
At some point during J.D.'s first two years, and maybe the beginning of his third, he saved Mr. Booker's life. He then took Mr. Booker to his high school reunion to brag that he was a doctor. Apparently they had fun together at the reunion until Mr. Booker urinated in J.D.'s rental car. ("My Friend the Doctor")