Use jQuery to create a cookie that modifies the viewport so that you can disable responsive design on devices.
Very simple to use. Place jQuery in the head of your page, then place desktopMode.js before header to avoid page stutter loading--loading once with initial viewport settings, then changing them.
Use the following link wherever you want to place the toggle:
<a href="#" id="view-toggle" class="view-desktop">Desktop Mode</a>
Should be good to go.