Render a node as plain text for sending by mail
The render engine of Drupal cannot render Node as plain text (at least natively).

I think you have to choice between send the mail in HTML format with is very easy by changing the $message['header'] of your mail or installing Mail system with Swift Mailer (very easy too). Or you have to parse the field value yourself by escaping HTML markups (bad way IMO).

I strongly recommend you to send your mail in HTML format if you want to enjoy the benefits of the drupal render engine, which is made for generate HTML output.