首先说，这是个被动结构，如果换成主动结构比较好理解。
(The judge)（主语） has found （谓语）him （宾语）guilty（宾补）.
换成被动后，宾语变主语，所以宾补变成了主补。
He（主语） has been found（谓语） guilty（主补）。
类似用法：
主补：对主语的补充。
The bottle was found empty. 人们发现瓶子是空的。（主语补足语）
The glass was found broken.
The door was painted white.
He was elected monitor.
She was found singing in the next room.
He was advised to teach the lazy boy a lesson.