This is essentially the issue of the Friends of Friends post. In this case, I am not really interested in expanding the audience.
Say I publish a friends only post. Victor, my friend, makes a comment tagging Roberta, not my friend, and asks a question directed at her. She is not notified about the tag. Nor can she see the comment or post.
Therefore, in my mind, allowing the tag to be done is counterproductive. Facebook should warn Victor that Roberta cannot see it. Ideally it would be ahead of time and prevent it. Less acceptable, but I would be happier is after the fact having a “Roberta cannot see this” notice. (The “Who can see this?” thing is vague and not generally very helpful clarifying exactly who can see it.)