No. It's not proven. Just a court approved diagnosis.
Well this diagnosis is sufficient to have her starved to death.
On the basis of two doctors chosen by the husband and an openly pro-euthenasia doctor chosen by the court, who investigated Terri for less than an hour each, IIRC.
How about letting Terri be examined for a week by someone who doesn't want her dead and can ensure that she's not being drugged?