There's nothing he can really write that any court would have to listen to - after all, from the court's perspective, he could have been lying when he wrote it down. And following orders is generally not a defense (although if he was under actual duress, like they were threatening to kill him if he didn't do it, that could be a defense.) But being a pawn rather than a mastermind can impact the sentence.
If the crime is a federal one, the Federal Sentencing Guidelines adjust the offense level according to the role in the crime:
§3B1.1. Aggravating Role
Based on the defendant's role in the offense, increase the offense level as follows:
(a) If the defendant was an organizer or leader of a criminal activity that involved five or more participants or was otherwise extensive, increase by 4 levels.
(b) If the defendant was a manager or supervisor (but not an organizer or leader) and the criminal activity involved five or more participants or was otherwise extensive, increase by 3 levels.
(c) If the defendant was an organizer, leader, manager, or supervisor in any criminal activity other than described in (a) or (b), increase by 2 levels.
§3B1.2. Mitigating Role
Based on the defendant's role in the offense, decrease the offense level as follows:
(a) If the defendant was a minimal participant in any criminal activity, decrease by 4 levels.
(b) If the defendant was a minor participant in any criminal activity, decrease by 2 levels.
In cases falling between (a) and (b), decrease by 3 levels.
The lawyer would probably not qualify as a "minimal participant" if he's writing reports detailing the activity, as this shows he's aware of the extent of the illegality. He would, however, probably qualify as a "minor participant".
If the crime was a state crime, then according to Oregon rules:
(1) Subject to the provisions of sections (2) and (3) of this rule, the following nonexclusive list of mitigating and aggravating factors may be considered in determining whether substantial and compelling reasons for a departure exist:
(a) Mitigating factors:
(D) The offense was principally accomplished by another and the defendant exhibited extreme caution or concern for the victim.
(E) The offender played a minor or passive role in the crime.