I actually think Franks private bill argument is correct. But is this bill get a temporary injunction to feed Terri, it will serve its function even if later declared unconstitutional.
But in the mean time it would be better if a law of general applicability is passed to give Terri and others more permanent protection.
By the plain wording of Amendment XIV, Congress' power is extremely broad. Broader than it should be, most likely, but if someone is being put to death without having committed any crime, that would be a violation of Amendment XIV; if Congress finds that such an action is taking place, XIV.5 would seem to empower them to pass legislation to stop it.