It may depend on what version of IE they are using...
Look in your theme admin area for an option for the IE user string:
Output example (found in source code in <head>):
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7"/>
or
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8"/>
or
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE9"/>
Or add one to your theme's header.php file.
One can also read the browser agent and load content specific to that browser of course.
Site looks the same in IE9 as Chrome or Firefox to me.