if (richTextBox1.Text != "")
        {
            mainForm newMainForm = new mainForm();
            newMainForm.dataGridView1.ColumnCount = 3;
            newMainForm.dataGridView1.Columns[0].Name = "Product ID";
            newMainForm.dataGridView1.Columns[1].Name = "Product Name";
            newMainForm.dataGridView1.Columns[2].Name = "Product Price";

            string[] row = new string[] { "1", "Product 1", "1000" };
            newMainForm.dataGridView1.Rows.Add(row);
            row = new string[] { "2", "Product 2", "2000" };
            newMainForm.dataGridView1.Rows.Add(row);
            row = new string[] { "3", "Product 3", "3000" };
            newMainForm.dataGridView1.Rows.Add(row);
            row = new string[] { "4", "Product 4", "4000" };
            newMainForm.dataGridView1.Rows.Add(row);
        }

subForm sub = new SubForm();
sub.Show(); // or however you have done it

private mainForm _parent;

internal void RegisterParent(mainForm form)
{
    // error handling of course, make sure form is not null, or you don't already have a parent
    this._parent = form;
}

sub.RegisterParent(this);

if (richTextBox1.Text != "")
    {
        _parent.dataGridView1.ColumnCount = 3;
        _parent.dataGridView1.Columns[0].Name = "Product ID";
        _parent.dataGridView1.Columns[1].Name = "Product Name";
        _parent.dataGridView1.Columns[2].Name = "Product Price";

        string[] row = new string[] { "1", "Product 1", "1000" };
        _parent.dataGridView1.Rows.Add(row);
        row = new string[] { "2", "Product 2", "2000" };
        _parent.dataGridView1.Rows.Add(row);
        row = new string[] { "3", "Product 3", "3000" };
        _parent.dataGridView1.Rows.Add(row);
        row = new string[] { "4", "Product 4", "4000" };
        _parent.dataGridView1.Rows.Add(row);
    }