internal FlyGrid.Net tooltip it is difficult to use for these purposes, as any mouse move will hide this tooltip.
But you can use standard System.Windows.Forms.ToolTip component:
[VB.Net]
Private Sub dgParams_NodeCellChange(ByVal sender As System.Object, ByVal node As NineRays.Windows.Forms.Data.NodeBase, ByVal index As Integer, ByVal value As System.Object) Handles dgParams.NodeCellChange
If index = EParamFields.e_controlField AndAlso Not value Is DBNull.Value Then
Dim toolTip1 As New ToolTip()
toolTip1.SetToolTip(Me.dgParams, "test")
toolTip1.Active = True
End If
End Sub