It is very difficult to deal with this via a third person.
First item is to check forum permissions for guests, users(default groups) and any other groups they may be assigned to.
Also I assume the board has not been modified from stock.
But if I had to guess first permission would be at least can see forum,Can read topics .
Or at the very least in forum settings,
Is Teaser Forum?
Forum will be visible in the forum listing even if the user does not have permission to read the forum.
Show forum in Active Topics- should be enabled (probably the culprit)
This may have been disabled because they could not figure out how to stop access.
Using the steps posted above.
