The whole phrase is this: ..Dan Cody sober knew what lavish doings Dan Cody drunk might soon be about.. The adjective 'sober' isn't qualifying the verb here, it's qualifying the noun 'Dan Cody'. The noun phrase of this clause is 'Dan Cody sober'. This is an unusual situation where the adjective actually comes after its noun. Another way of saying this is 'When sober, Dan Cody knew ...' or 'When he was sober, Dan Cody knew ..'