Try setting this property and subscribing to this event.
this.gridControl1.Model.Options.ResizeColsBehavior = GridResizeCellsBehavior.ResizeSingle;
this.gridControl1.ResizingColumns += new GridResizingColumnsEventHandler(gridControl1_ResizingColumns);
private void gridControl1_ResizingColumns(object sender, GridResizingColumnsEventArgs e)
{
if(e.Reason == GridResizeCellsReason.MouseMove)
{
this.gridControl1.ColWidths[e.Columns.Left] = e.Width;
this.gridControl1.ResetVolatileData();
e.Cancel = true;
}
}