Well, the short answer is... Nothing. Their post is gone; you can't do anything to it. Who knows if they'll ever come back?
Of course, if they had other posts on the site you might be able to leave a comment on one of them and help out that way. This could come across as somewhat stalkerish though, so don't over do it. And there's still the chance that the author has left and won't come back...
If they have a chat account, you could try inviting them into a room and letting them know that way. Unless, of course, they've completely left with no intention of ever returning.
In cases where the question isn't a duplicate, leaving an answer can prevent the question from being deleted by the author, if it gets an up-vote. This won't necessarily mean the author sees your corrections (if they've left, never to return, they won't), but it might at least help someone else. Of course, in the situation you're referencing, it was a duplicate.
FWIW, in this case the post you linked to appears to have received an upvote right about the time the post was deleted... So you may have been more helpful than you realized.