You should use either PrepareViewStyleInfo or Model.QueryCellInfo to do this.
If e.RowIndex > 0 And e.ColIndex = 5 Then
If me.Grid(e.RowIndex, 2).Text = "???" And me.Grid(e.RowIndex, 3).Text = "???" Then
e.Style.Enabled = False
end If
End If
I would try this code in PrepareViewStyleInfo to see if it works there. If not, then it shoul dwork in Mode.QueryCellInfo. Here is a thread on a similar request.
http://www.syncfusion.com/Support/Forums/message.aspx?MessageID=15181