Create a string property in your second form, that will hold the textbox contents. Then in your Main form you need to hold a reference to the second form. Then when your DataGridView cell changes you can update the string property you created, and bind the textbox to that string:
something like this
Public Class Form1
Private myForm2 As Form2
Private Sub DataGridView1_CellLeave(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellLeave
myForm2.textBoxResult = DataGridView1.CurrentCell.Value
End Sub
End Class
Public Class Form2
Public textBoxResult As String
End Class
Hope this helps