Unfortunately, currently this is "compiled" into treeview.
If you own FlyTreeView source code, you can simply put the following JS code in the beginning of fn_Node_MouseDown function (node_07_handlers.js file):
// exit if not button 0 (IE button 1) is pressed
if (!ch_fromKeyboard && ch_event &&
((vr_b_ie && ch_event.pt_button != 1)||(!vr_b_ie && ch_event.pt_button != 0))) return;
We can make this a global option (e.g. FlyTreeView.RightClickSelects), but this will take some time.