There is no events that is thrown when you make a post visible.
What you can do : You can subscribe this.allPosts.ItemCommand inside \Sitefinity\Admin\ControlTemplates\Forums\ControlPanelViewThreadTemplate.ascx
or its code behind.
Then you can check the check the CommandName which should be ChangeModStatus and get the post Id form e.CommandArguments. Then send an email.
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