Sounds like the original designer uploaded index.html, then decided to change it to index.php and forgot to delete the original from the server. There's no advantage in having both, only pure misery! Since you want the left column to appear on every page (including the home page) you definitely want index.php only.
Next step is to create an include. There are various methods, but what I recommend is this:
Whatever left column code you want to appear on each page (be it the whole column or just a certain part), cut out the code for that and paste it into a new file called leftcol.php. Place this file in the /includes/ folder.
Then, in place of that code on each page, put this:
<?php include $SERVER["DOCUMENTROOT"] . "/includes/leftcol.php"; ?>
Now the content in leftcol.php will appear on each page, no matter where it is on the site. So when you want to update the content, open that file and update it there.
Just a side note. At some point you need to let the web designer know that you are doing this. Otherwise, if he/she makes changes to your site via ftp, they will overwrite anything you have done. They first need to download your changes before making any further changes. (I've seen this conflict happen, to the great misery of all...)