WORKAROUND:
radListView1.Groups[0].PropertyChanged += Form1_PropertyChanged;
..............
 void Form1_PropertyChanged(object sender, PropertyChangedEventArgs e)
 {
     if (e.PropertyName == "Expanded")
     {
         ListViewDataItemGroup group = (ListViewDataItemGroup)sender;
 
         RadMessageBox.Show(group.Text + " is expanded = " + group.Expanded);
     }
 }