According to the documentation
ColumnWidthProvider This property can hold a function that returns the column width for each column in the model.

Not Working - I defined method called columnWidth & gave method name. This did not work.

Working - If I define method directly with property value, then it works. e.g uncomment the following commented code & comment another line.

Based on the above documentation, I did the following.
Code snippet below.

Window {
   id : root
  function columnWidth(column)
{         console.log(" Value...."+column)         return 50     }
    HorizontalHeaderView {
        id: horizontalHeader
        width: parent.width
        height: 100
        // columnWidthProvider: function(column)
{         //        return 50;         //    }
        columnWidthProvider : root.columnWidth(column)
      }
}