Nah, I figured it out. It was the clothes she had on that disappeared, not her robes. I just reverted to a game I had saved just before talking to her, checked her equipment, then dueled her. It fixes the problem, it seems...
"My word is my honor. My honor is my life."
-- Raphael Griffinhart, Demonchild, Angelkin, Death Incarnate, the Final Warrior