One way I think you can do this is to handle the TableControlCurrentCellInitializeControlText, and there format the string that is used to initizize the control text. Here is a little snippet. You would have to ID the cell somehow where you want this formatting to happen. Here the code just formats all doubles.
private void gridGroupingControl1_TableControlCurrentCellInitializeControlText(object sender, Syncfusion.Windows.Forms.Grid.Grouping.GridTableControlCurrentCellInitializeControlTextEventArgs e)
{
if(e.Inner.CellValue is double)
{
e.Inner.ControlText = string.Format("{0:F2}", e.Inner.CellValue);
}
}