I was playing the Heroic Route, took Sarah out on her second date and triggered the unwinnable fight. Managed to confuse the boss after about 4 casts, and then it killed itself. Cassandra suddenly showed up, which I assume is due to the Submissive Route letting you win that fight. Not exactly sure if this is the right place for this, but I didn't find a bug report box anywhere.
Edit: Took a look at my party, looks like Cassandra is permanently locked in, despite never having interacted with her before this point in the game.