does she see a pdoc and T for her MI? if so,here's a thought: you might suggest she keep a journal to discuss those issues she brings up to you in her next session since you aren't qualified to give her help other than be a supportive friend.
sounds like to me she is obsessing over her illness and it might be she is taking you hostage over what ails her. idk. just a thought.
tell her she is valued as a friend but using you as a sounding board for everything may not help her...that's what her pdoc and T are for.
you can say this kindly and if it were me i'd bring the subject up to her.
i might be more prone to say just be a good friend but from your post it sounds like it's interfering with you enjoying having her around since she's yaking on and on about herself and her MI. it's ok to set boundaries with her. that's a healthy approach.