You can use MouseDown event to detect left or right mouse button is pressed:

if Button = 1 && left button
&& your code goes here
endif
if Button = 2 && right button
&& your code goes here
endif

In this code Button is passed parameter in MouseDown event. You can hold button in variable and use it in NodeClick event.

In VB events are fired in this order - MouseDown, NodeClick, MouseUp. In VFP there are little problem - if you click on the node, MouseUp event will NOT fire. Therefore, in VFP you cannot use MouseUp event for this detection.