The best solution (IMHO) doesn't actually work: you should be able to use the [Tokens][1] module and use a proper token to fill a recipient field. For example:
`[node:author:mail]`

However, Webforms only appears to support its own *%something* tokens, and those only relate to the current user and the webform id (not the id of the node being read).

So instead, write the site's inevitable custom module and use [`hook_mail_alter()`][2] to change the To address. 

Another resource I read (and have now lost) suggested setting the Webforms recipient to something easily recognizable, like "[email protected]". 

Then in your `hook_mail_alter()` function, 

 1. confirm the form's identity, 
 1. get the current post's author, 
 1. get that user's email address, and 
 1. use it in the Recipients. (Along with the admin & the poster, as you like.)


  [1]: https://drupal.org/projects/tokens
  [2]: http://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_mail_alter/7