In my experience, this kind of thing almost always ends up becoming simpler one way or another after some time passes, for better or for worse. But obviously, sitting around and expecting things to resolve themselves isn't a very intelligent way of going through life. So I'd say gather more information. Start with talking to your old roommate about their autoimmune problem, as that seems to be the path you want to go down. See if it's a problem that could be easily worked around(since I obviously know nothing about her.) I can already tell you though that your friend with the mental issues, going down that path will not help either of you, neither in the short nor long run. That said, nobody said you can't keep in touch with her. And it wouldn't be a terrible idea to have a serious conversation about this with her, so you don't blindside her(since that's the last thing she needs, obviously.)
It all boils down to that you need to have conversations with all these people.