I'd say start Very slowly with the exercise---simple fact if she can't do five minutes of stretching without damaging herself, she should do a three-minute workout. Start with something non-stressful---there are a lot of good qigong styles that demand zero strength and balance and can help the body get back on track with itself.
A lot of this is mental, I suspect---fear and frustration expressed as exaggerated physical reactions. Qigong can help with that, or some counseling, ot even a good online chat room, so long as she can tolerate the rest of the stuff that comes with the rare gem of good advice.
Once she thinks she can exercise without getting hurt, she can try walking---five minutes every other day this week, ten next week, whatever. Do less than possible because a lot of the initial health benefit will be mental.
I understand the depression-food death-spiral, but if you can help her break that ....also a benefit.
Unless she has some whole-body degenerative disease, she can get healthier .... it might start really slowly, but it is simply physiology.
The mental aspect is generally overlooked. She can literally hurt herself due to feat and expectation of injury---tensing up and pushing a little too hard simultaneously, because part of her brain is trying to defend her, part is trying to extend her, and part is trying to convince her that disaster is imminent.
She really needs to get her head together if she wants her body to work. The brain is by far the most powerful organ.