We don't know what Washington or Lee's own Federalist father "Light Horse Harry" Lee would have thought of Robert's turning his back on his oath and joining the Confederate forces.
We do have the example of Winfield Scott, George Thomas and Lee's cousin Samuel Phillips Lee, Virginians who honorably served the Union, though.
Nor do we know what Lighthorse would have thought of a union which would do something so horrendous to Virginia.
Of course Harry died before Robert was old enough to have adult discourse on politics. Perhaps that would have made a difference though IIRC Harry had been disappointed in many later Federalist policies (tariffs come to mind) and with the hindsight of age all men see intemperance in their past- so he may not have defended the union as strongly as his history suggests.
Yet I feel sure, though he lived a thousand years, he would have always been appalled at any criticism of General Washington!