I think un-deleting a post by the non-author should be done only where the author is vandalizing their own content and the post still has value for the site and the future visitors. In a situation like this one, I think "forcing" the user to receive (and accept) an answer is not the appropriate approach. What I suggest is that you duplicate the question, mention that it is a duplicate of a deleted one, and provide the answer to your own question.
Then in a couple of weeks, just remove the part that it is about a deleted one because no one will care at that moment.