As is often the case, the point has floated past you.
The porn matter is a by-the-by. If he wants to watch on his own computer and in his own time, that's his business.
It is the context of the event(s) and the lack of integrity inherent in his behavior and the attempts to both dodge consequences and maintain employment in a situation where integrity is not just a like-to-have, it is imperative.
If he can't monitor his own, he has no right to sit in judgement on others.
We are assuming they didn't know. I find it hard to believe, even though the Law Society would be one of the better agencies to maintain privacy, that someone in the RIB didn't know.
BTW the main issue that undid him with the Law Society wasn't the misconduct itself which was minor but the fact that he didn't have the policy, processes and procedures in place to handle workplace misconduct as required by the Law Society. This point seems to fly past most online commentators. In my opinion that is the reason he probably shouldn't be employed at the RIB.
I can't work out why the Office Manager(s) didn't have this covered after 7 years. Everything was all good until the employer/employee relationship broke down then the complaint was made. I've seen that happen in employement cases many many times.