Okay, open admin/module/PIREPAdmin and paste the following in the approve_pirep_post() function at the very end. I think it should go through:
# Send an email to the pilot who submitted that PIREP
//PILOT INFO
$pilotfirstname = $pilotinfo->firstname;
$pilotlastname= $pilotinfo->lastname;
$email = $pilotinfo->email;
//PIREP INFO (This all goes into the email, we will need to add those into the TPL
$code = $pirepdata['code'];
$flightnum = $pirepdata['flightnum'];
Template::Set('pilotinfo', $pilotinfo);
Template::Set('flight', $pirepdata);
$sub = 'Flight Report for: '.$code.''.$flightnum;
$message = Template::Get('email_pirep_accep.tpl', true);
Util::SendEmail($email, $sub, $message);