If you want to do this, you're going to have to dynamically change your viewport tag based on the width of the screen.
The viewport
meta tag is what makes responsive design possible on mobile devices by forcing the browser to use the device-width
as the width for the viewport. By removing this restriction, most mobile devices will lie about their width in order to get the full site. With javascript you can dynamically change it based on the page width
Start by including the tag in your HTML head
element:
<!-- language: lang-html -->
<meta name="viewport" content="width=device-width, initial-scale=1.0">
Then we'll listen to the resize event and change the viewport according to your business rules:
<!-- language: lang-js -->
$(window).resize(function() {
var mobileWidth = (window.innerWidth > 0) ?
window.innerWidth :
screen.width;
var viewport = (mobileWidth > 360) ?
'width=device-width, initial-scale=1.0' :
'width=1200';
$("meta[name=viewport]").attr('content', viewport);
}).resize();
##Here's a working Demo in Fiddle
Note: Desktop browsers ignore the viewport tag so this will not have any affect on your development machine. In order to test these changes, you can open up the chrome debugger tools and emulate a mobile device.
###Here's a screenshot:
That all being said, I would work with your user to advise against this.
Here's a quick article I just threw up on Adding a Desktop Mobile Toggle Button
It's generally not a good idea to prevent the end user from doing something that they want to do. Your first goal should be to develop a site that works well on small devices. If a user really wants to see a zoomed out view, then you can provide a toggle switch for them at the bottom of the page.