The simplest option would be to send the email to the user and they could forward it.

Or, if you're giving them an HTML page you could include a hyperlink of the form <a href="mailto:user@domain?subject=Blah&body=Blah%20Blah">Send Email</a>.

Or, (once again assuming an HTML interface) you could give them the email body in a textarea in a form. When they submit the form (presumably after editing the text), your script would use Mail::Sender or similar to send it.