Ok, here the answer from support which worked fine for me:

The only way to modify the tooltip for any tool in the UltraToolbarsManager is to use its SharedProps.ToolTipTextFormatted property, but the tools don’t implement Show or hide functionalities.

There is property “ShowToolTips” for the UltraToolbarsManager, Groups, which we can use to hide the tooltips temporarily in BeforToolActivate event handler:

private void ultraToolbarsManager1_BeforeToolActivate(object sender, Infragistics.Win.UltraWinToolbars.CancelableToolEventArgs e)

        {

            if (e.Tool.Key == "ButtonTool1")

            {

                //this.ultraToolbarsManager1.Ribbon.Tabs[0].Groups[0].Settings.ShowToolTips = Infragistics.Win.DefaultableBoolean.False;

                ultraToolbarsManager1.ShowToolTips = false;

            }

        }

        private void ultraToolbarsManager1_AfterToolDeactivate(object sender, Infragistics.Win.UltraWinToolbars.ToolEventArgs e)

        {

            //this.ultraToolbarsManager1.Ribbon.Tabs[0].Groups[0].Settings.ShowToolTips = Infragistics.Win.DefaultableBoolean.True;

            ultraToolbarsManager1.ShowToolTips = true;

        }