His other order had statutory authorization, the statutes of which were posted here. If his order forbidding oral food and hydration is valid, it should be possible to cite a statute allowing it.
Note that before the gastrostomic feeding was stopped, Greer's order forbidding oral feeding and hydration might have been legally allowable on the basis that such attempts would pose certain health risks. Such a basis for prohibiting such feeding vanishes, however, if the patient has no other means of receiving food and water.
I believe the feeding tube was pulled at the time. That is why they tried to feed her.
"A year later, in April 2001, her death by dehydration was ordered to begin, and the day her feeding was stopped her brother and sister came by with a spoon and a cup of pudding, asking a nurse to try to feed her by mouth. The nurse refused and reported the request to others. When Schiavo found out he demanded that Bobby and Suzanne be removed from the list of approved visitors, and Greer rubberstamped his request"
Brother and sister tried to feed Terri in 2001 after feeding tube pulled.