There are multiple options to fix that forum. We have no ability to do anything without contact with the owner though.
I can think of multiple things to attempt to try and fix it. Looking through the code on github, it doesn't look like they are writing tests for their software, which would make diagnosing the database a bit easier.
How I would probably proceed would be to setup a clean install of the forum on a different server, make note of the database schema, and then compare that to the schema on the corrupted database. If they match, I would probably start by writing a script to copy over information one piece at a time, testing each time to see that the forum is not corrupted. If it is corrupted, that will tell you where the problem is. This would also have the benefit of removing any possible corruption caused by addons/plugins or modifications made over the time the forum has been active, restoring the users, messages, posts, etc. to a pristine state.
If that doesn't work, which it should, I would restore the database on the test server from the live server, and then I would write a script to delete posts one at a time, starting from the most recent post, and then test to see if the problem is repaired. A script to do such a thing would probably take about half an hour or less. If no problem is found, reset the database and start deleting messages one at a time, starting from the most recent, and see if one is causing the problem. If no problem is found, reset and start deleting users one at a time and see if it's a user that is causing the problem.
Anyway, I would be more than willing to help, as I said, and I wouldn't even charge for it. Repairing corrupted databases isn't usually terribly difficult, unless someone has fiddled with the corrupted database too much. In the latter case, I would hope he backed up the database prior to making those attempts to fix it.