This does not look to be simple thing to do with this treeview.
But actually there's a kind of trick.
There're some lines in node.htc file:
function o112(){
if (o75() || o96() == 0) return;
if (o46) {
TreeView.queueNodeEvent(element, 'ONBEFORECOLLAPSE');
This is actually Collapse function of node and you may try to check the Key value in order to prevent node from collapsing (o42 is a Key property as you may refer from the header of node.htc):
function o112(){
if (o42 == "yourRootNodeKey") return;
if (o75() || o96() == 0) return;
if (o46) {
TreeView.queueNodeEvent(element, 'ONBEFORECOLLAPSE');
P.S. This looks to be much easier if you have source code of the control.