How to add someone to a hidden conversation on Facebook?

[Q]

My friend posted a status that was only visible to a few of us for scheduling, but he forgot one person. He can't figure out how to add her. Will she be able to see the conversation if I tag her in a comment or does he have to do something?

[pri✍]

Your friend who posted the status can change the audience setting to add the forgotten one, by clicking again the audience setting ➤ Custom, add the name of the forgotton to the 'who can see it'.

I don't think tagging will do, as the rule govern that post is just for certain people to see it. Why don't you try that out to prove it?