TextBox tb;
private void dataGridView1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
    if (e.Control.GetType().Name == "DataGridViewTextBoxEditingControl")
        tb = (TextBox)e.Control;
}

private void button2_Click(object sender, EventArgs e)
{
    if (tb != null)
    {
        int index = tb.SelectionStart;
        dataGridView1.CurrentCell.Value = tb.Text.Insert(index, this.button2.Text);
        tb = null;
    }
}