Provided that the right side of the page is an IFRAME, the most appropriate solution in this case would be to define a function on the main page. The function could take some data as parameters and update the TreeView with it.
Later call this function from the IFRAME when it is necessary.
I hope this gets you started.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.