Her Act 3 quest opens up to completion but only if she is the designated protagonist for the playthrough which legitimately baffles me as it just dead ends for no apparent reason otherwise. Personally I feel this makes no sense thus I'm skeptical. This might also be the case for other companion stories as well and barring the lack of epilogue slides for anyone, would explain the disappointing conclusions some people have had for companion's stories.
Edit: If true that is a stipulation I find should be removed.
Yeah, the lack of epilogue slides damages potentially good endings, and her resolution when you play as her is better but still wanting..