Who is Bill Harper from Johnny Got His Gun and what is their importance?

Bill Harper is Joe Bonham's best friend in Shale City, Colorado. Harper ostensibly tells Bonham the truth about girlfriend Diane going on a date with rich Glen Hogan, and is punched for his troubles. Bonham and Howie leave town to work laying track in the Uintah desert to get away, but return to find Harper and Diane kissing on her porch.