Templates are basically HTML with some Freemarker script (that is readable even if you don't know Freemarker). If you haven't found it, the best place might be the post_show_action_buttons_inc.htm file which displays the subject line, etc.
The quickest way is to do a mailto: link on the page. Something like:
When clicked the user should have an e-mail message opened up in his PC with the Subject line being a URL to the post in question.
Alternatively, you could use something like JFormMail with is a
Servlet that takes the output of a form and turns it into an e-mail. That way you could have the link point to a form that specified the specific info you want.
[originally posted on jforum.net by monroe]