It opens the door to EVERY SINGLE person on a feeding tube in this country this very minute to go through this same process; the Congress will now decide who lives and dies.
Whether or not we agree with the decision her husband made, she's had more "due process" than any living party to an ongoing case.
No, even in this case the courts are deciding and Congress is second-guessing. If you are correct about your assessment of the separation of powers and the federal involvement in a state case, then Congress's law (if they can pass one) will be shot down in federal court. Many people here won't be happy about that, but that will be the end of it IMO.