<?php echo get_post_meta($post->ID, 'client_logo', true); ?>

( function( $ ) {
// AJAX loading client information
    var page = 1;
    var loading = true;
    var load_posts = function(){
            $.ajax({
                type       : "GET",
                data       : {'What should be here?'},
                dataType   : "html",
                url        : "link to php file which is return data? what should be at the php file?",
                success    : function(data){
                    } else {
                    }
                },
                error     : function(jqXHR, textStatus, errorThrown) {
                    alert(jqXHR + " :: " + textStatus + " :: " + errorThrown);
                }
        });
    }

    // Load more posts on click
    $('a.popup').click(function() {
      loading = true;
      load_posts();
      return false;
    });
    load_posts();
} )( jQuery );

add_action("wp_ajax_clientLogoPopup", "clientLogoPopup");
add_action("wp_ajax_nopriv_clientLogoPopup", "clientLogoPopup");

    function clientLogoPopup()
    {
        $clientID = intval($_REQUEST['client_id']);
        if($clientID > 0){
            echo get_post_meta($clientID , 'client_logo', true);
            // or you can also show other meta fields here.
        }
        die();
    }