Thank you for your question today, I look forward to assisting you. I have nearly 20 years of legal experience in various disciplines, including JAG.
Unfortunately, there just isn't much that can be done.
If a soldier in the Army has less than 6 years of service (and I'm assuming he does due to his rank), he is not entitled to a board hearing. The only other way he would be entitled to a board is if command was seeking an Other than Honorable.
Now, you stated that they were seeking an Other than Honorable. I believe you have your terminology wrong here. An Other than honorable is not available under Chapter 13. Only Honorable or General (under honorable conditions). Para 13-10, AR 635-200 says as much.
So, he will end up with a General Discharge. His rights are limited. He can submit matters on his own behalf to his commander and the commander that will be the approval authority. If the discharge is approved, he can appeal to the next level commander who can either disapprove the discharge, change the status to Honorable or allow the discharge.
Beyond that, there is no appeal right and nothing else that can be done. This is an administrative and not a judicial process, so due process is not legally required beyond the regular notification, allowing him to submit matters on his behalf and the appeal to the next level commander.
If you wish to help, you can certainly submit a statement on his behalf, explaining that this is one issue that was corrected during the rehabilitation period, so you do not believe this is a true lack of performance or pattern. Point to the reduction in rank, stating that an appropriate punishment has already been accomplished.
That's really all you can do here.