how to cancel fancybox from an AJAX callback within onStart

Question

I am loading a login form within a fancybox. If the user has the site open in two browser tabs, and logs in in the first tab, and then clicks the login link in the second tab, I want to avoid opening the fancybox in the second tab since the page will redirect to the user profile and won't look good inside of the fancybox. So I have an AJAX callback within the onStart setting to check if the user is authenticated. If the user is already authenticated, I want to cancel the fancybox and direct the parent to another page (such as the user profile).

Here is my onStart code at the moment:

'onStart': function(opener) { 
   if ($(opener).attr('id') == 'login') {
     $.get('/hicommon/authenticated', function(data) { 
        if ('yes' == data) {
          console.log('this user must have already authenticated in another browser tab, SO I want to avoid opening the fancybox.');
          return false;
        } else {
          console.log('the user is not authenticated');
          return true;
        }
     }); 
   }
},

This doesn't prevent the fancybox from opening for authenticated users, presumably because I'm returning false within the AJAX callback, rather than in the scope of the onStart function. Is there some way I can cancel the fancybox from within that AJAX callback, or pass something to the scope of the onStart function? Thank you