When a Trombone player read music from the Treble Clef How does he/she reads it, does he/she reads middle C an Octave down?
There are two possibilities. He may be reading "brass band treble", a major 9th higher than sounding pitch. In this case the part would be in treble clef throughout, and the musical context would be a British tradition brass band. Or he may be reading a concert pitch part which has ventured into treble clef from its usual bass clef.
There's no adjustment for Octave. Middle C is still C4.