Melissa, if the client is using an outdated version of IE, there's a good chance his browser is messing with HTML5 elements it doesn't understand (like <nav>).
You can add code snippets to your page to make all browsers recognize these elements, or you can change your code to something that already works (<div id="topnav">, for example).
When you open a new blank page in the HTML Editor, notice the code in the head includes the HTML5 shim. That's there to prevent these weird browser renderings.
<!--[if IE]>
<script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script><![endif]-->
You should also ask what version of IE the client is using. If it's below 9, well – your site isn't the only one he'll be seeing wonky things on. You could try, with as much subtlety as possible, to let him know that browser upgrades are FREE
Here's a good resource:
http://html5boilerplate.com/
Clients

it's hard when they "know" more than we do!