You (instead of creating a window or a dialog) should use a simple
div
element, and inside you can load the content of your User control. I have always said, that ASP.NET code once streamed down to the client is in the form of HTML always. So, there is no need to worry of how you content would look like, it will be simple HTML markup with a few classes ids and other attributes.
You can create a simple div element, such as
<div id="user-control"></div>
Done. Now, you can load the data from the server's web page where your control is located at, and you can use jQuery's ajax or get loaders to get the data. I would prefer using the get loader, or load to fill the element with the content from the server.
$('#user-control').load('<url-of-the-control>');
This would load the content of your User control's HTML markup into the div element. You can after that use the CSS styles to show the div as a floating window; more like a dialog box. It now then would be a simple HTML document's element. That can be easily edited using CSS, or jQuery itself.
More on jQuery's
load function[
^].