Only just had a chance to look into this, it's a very simple fix. I have changed this for the next update.
Line 641 of library/FriendInviter/ControllerPublic/FriendInviter.php is:
Change it to:
(Should be https instead of http).
Line 641 of library/FriendInviter/ControllerPublic/FriendInviter.php is:
PHP:
$twitterLogin = "http://api.twitter.com/oauth/authorize?oauth_token=$requestTokens[oauth_token]";
Change it to:
PHP:
$twitterLogin = "https://api.twitter.com/oauth/authorize?oauth_token=$requestTokens[oauth_token]";
(Should be https instead of http).