The subject on emails sent through the Tell a Friend process show "Order Confirmation" as the subject line.
The file /includes/languages/english/tell_a_friend.php shows this:
define('EMAIL_TEXT_SUBJECT', 'Your friend %s has recommended this great product from %s');
I copied that file to my /includes/languages/english/CUSTOM/tell_a_friend.php and made a change to the subject line (even though it already shows what the correct subject should be). No effect. I then changed some of the other info in the body of the email. Those changes took effect.
After searching the forums, I used the Developer's Tool Kit and searched for _subject in "CONSTANT or Language File defines" and for Order Confirmation in the "Look-up in all files". Didn't see any other files that would impact on the subject line of Tell a Friend emails.
Usually by the time I've finished typing out a question here, I have managed to find the answer for myself. Not so this time Help?