You can use the SelectionChanged event of the RadTreeView and set the Content of the RadDropDownButton.
If you tree is declarative you will have to build this string path by using the RadTreeViewItem.Parent property.
If the tree is bound to collection of ViewModels/BusinessObjects , you can create function that builds this path recursively. You can examine similar approach in this blog post.
Please let us know if you need further assistance.
the Telerik team
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