You need to use the ValueValidating event. This is an extract from the docs
The Editing Lifecycle
A node enters edit mode
A node that is being displayed by the RadTreeView control is selected and the user presses the F2 key to bring the node into edit mode.
The RadTreeView control calls the BeginEdit() method and a new editor instance is initialized. It is available publicly through the ActiveEditor property in RadTreeView and is associated with the node that is about to be edited.
The editor fires its Editing event, which in turn triggers the firing of the RadTreeView Editing event. If either event is canceled, no further action takes place.
A text box based editor appears for input.
A node is brought out of edit mode
The editor determines if it wants to handle the keystroke.
The editor instance performs the action it has defined for the Enter key. Typically this indicates that edit mode should be exited and any changes made during the edit session should be applied to the node Text property.
In response to the action described in the previous step the EndEdit() method is called and the ValueChanged event is fired.
The RadTreeView fires the ValueValidating event which allows the user to hook up custom logic for verification. If the ValueValidating event does not succeed (e.Cancel is true), ValidationError event is fired to notify all listeners that the validation has failed.
The RadTreeView control sets the node Text property to the string representation of the editor Value property.
The full documentatation on editing nodes can be found here
hope that helps