Wow, she may be a difficult person because of her mental illness, but it sure seems like she's surrounded by people who are just plain mean and inconsiderate to her. I'd say that the thing you need to work on, regarding her, is your compassion. But I would seriously consider what a douchey thing your SO is doing. Start taking a look at how he treats people, as I would see his treatment of her as a big red flag.
Independent solo polyamorist seeking lover-friends willing to invest in friendship, companionship, and love, but without a need for partnership.
Never confuse commitment with exclusivity, love with ownership, nor sex with intimacy!
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
~ Carl Sagan