Workaround:

        private void radGridView1_ConditionalFormattingFormShown(object sender, EventArgs e)
        {
            ConditionalFormattingForm f = sender as ConditionalFormattingForm;
            if (f != null)
            {
                RadPropertyGrid radPropertyGridProperties = f.Controls["radPropertyGridProperties"] as RadPropertyGrid; 
                radPropertyGridProperties.ItemFormatting += radPropertyGridProperties_ItemFormatting;
                  radPropertyGridProperties.ItemFormatting += radPropertyGridProperties_ItemFormatting;
            }
        }

        private void radPropertyGridProperties_ItemFormatting(object sender, PropertyGridItemFormattingEventArgs e)
        {
            if (e.Item.Label=="CellFont")
            {
                e.Item.Label = "aaa";
            }
        }