RadFormDecorator is able to decorate page content, which is refreshed with AJAX...
...but only when the page content update is made with the Microsoft Ajax framework (via the PageRequestManager client object etc). Of course, the Kendo UI Window does not rely on MS AJAX, so it does not perform AJAX requests via the PageRequestManager.
A possible approach in your case is to use the Kendo UI Window's refresh event and execute the RadFormDecorator's decorate() client method.
Another possible approach is to use the appendTo option of the Window and force the widget to be created exactly where you have placed it - this will be somewhere inside the ASP.NET form, while normally the Window would be moved in the DOM to become a child of the <body>, i.e. outside the <form>. Then, use an UpdatePanel inside the Window content and trigger updates via the UpdatePanel and not via the Kendo UI Window API.
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