The mock trial was definitely not a good idea. First, it was organized by Brinker, the conventional straight arrow and "big man on campus." All along, Brinker had lacked the sensitivity to understand the nuances of the relationship between Finny and Gene.
It was especially insensitive to spring the trial on Gene as a surprise for which he had no time to prepare, suggesting that Brinker's motives were at least partially malicious. Rather than trying to bring about healing, Brinker seemed to want to nail Gene in a public way as responsible for Finny's shattering fall.
Neither Gene nor Finny were ready to confront what happened, and certainly not in the guise of mock trial with its structured need for a guilty or innocent verdict. If we value human life at all, the mock trial was a disaster on an even more fundamental level, as it led to Finny's fall, surgery, and death.
Brinker should have stayed out of it and let Finny and Gene work out their issues in their own way.