Criminal defendants have the right to waive a jury trial, so I imagine it has happened at some point or another.
But we're not even talking about a criminal defendant here.
The one to have standing to request a jury would have been Terri. Her guardian, acting on her behalf didn't request one. Why would he, when the rulings were going the way he wanted them to go?
Meanwhile, her parents could never get standing, which is why their petitions for relief were denied.