SOLVED! There is actually no need to install any plugin.
The reason the other admin is not able to view the hidden forum is because bbpress has its own user roles and you need to assign the Keymaster Role to the admin in question.
To do that, simply click on Users on your WP admin menu, tick to select the user in question, in the tab that contains bulk actions, you’ll see “Change Forum Role to….” click it and select keymaster.
Then click on the change button to change it.
That should work the magic.
