add_action(
   'notify_friend_message_received',
    function(
        $friend_user,
        $message,
        $subject,
        $feed_url,
        $remote_url,
        $reply_to
    ) {
        // Your code here.
    },
    10,
    6
);

do_action( 'notify_friend_message_received', $friend_user, $message, $subject, $feed_url, $remote_url, $reply_to )

do_action( 'notify_friend_message_received', $friend_user, $message, $subject, $user_feed->get_url(), $remote_url, $reply_to )