private void FpSpread1_CellClick(object sender, FarPoint.Win.Spread.CellClickEventArgs e)
{
    if (e.ColumnHeader)
    {
        bool ascending = fpSpread1.Sheets[0].Columns[e.Column].SortIndicator != FarPoint.Win.Spread.Model.SortIndicator.Ascending;
        fpSpread1.Sheets[0].SortRows(e.Column, ascending, true, new NumericTextComparer());
    }
}

public class NumericTextComparer : System.Collections.IComparer
{
    public NumericTextComparer()
    {
    }

    public int Compare(object x, object y)
    {
        if (x == null)
        {
            return y == null ? 0 : -1;
        }
        else if (y == null)
        {
            return 1;
        }

        if (x is string s1 && y is string s2)
        {
            int index1 = s1.IndexOf('.');
            int index2 = s2.IndexOf(".");
            if (index1 >= 0 && index2 >= 0)
            {
                if (int.TryParse(s1.Substring(0, index1), out int n1) && int.TryParse(s2.Substring(0, index2), out int n2))
                {
                    if (n1 < n2)
                    {
                        return -1;
                    }
                    else if (n1 > n2)
                    {
                        return 1;
                    }
                    else
                    {
                        return s1.Substring(index1 + 1).CompareTo(s2.Substring(index2 + 1));
                    }
                }
            }

            return s1.CompareTo(s2);
        }
        else if (x is IComparable c1)
        {
            return c1.CompareTo(y);
        }
        else if (y is IComparable c2)
        {
            return c2.CompareTo(x);
        }

        return 0;
    }
}