Should not Terri also be a party, represented by a guardian ad litem who supports this law (as Wolfson, per public comments, does not)?
Possibly, and the judge might be more open to that. But it's still not necessary.
Essentially, the state is on trial here, accused of passing an unconstitutional law. The Attorney General is the best and most proper party to defend the state.
This trial won't be about Terri. It will be about the new law.