I've had some success talking to people from the perspective of an ex-vegetarian. Sometimes what we wish were true simply doesn't mesh with what our biology mandates, and that is a very hard hurdle to get over even when self self-discovered. It took me a long time to discover that what makes the brain happy makes the body happy, and it sounds like she hasn't discovered that yet, but might have the chance to if you can figure out how to intervene.
Get a copy of "The Vegetarian Myth" for yourself, and maybe offer it up to her as "an interesting read" just to open the dialogue. A The book goes into depth about how vegan diet can be responsible for just as much animal suffering as an animal based diet, it is just more behind the scenes. You can talk to her about the positive soil inputs of pastured animals, and how by getting her calories from grassfed beef, goat, or lamb she is reducing the world's dependence on fossil fuels, getting . Talk to her about the need for the right ratio of omega 3:6 for her brain to function properly (veganism is a nightmare for omega 6 ratios, and the lack of omega 3's and subsequent inflammation could be contributing to her dementia).
I also often bring up how we shouldn't confuse the "how" with the "what" in our diets. Vegetarianism is a just and noble response to the commercial meat industry, and that was pretty much all that was available in the '60's and '70's when the literature for vegetarianism was being written, but we now have free range chickens, and pastured beef and pork readily available, and properly raised animals don't cause health problems the way commercial meat does. So, from a health standpoint a veg. life doesn't hold water anymore. And from an ethical standpoint, all animals die whether we kill them or not, most guilt I suffer from taking a life of another being to sustain my own life has been mitigated by feeling some personal responsibility for that beings quality of life, and doing what I can to make sure it only has one bad day.
Let her know how impressed you have been with the progress she made improving her health , and detoxing from common elements in the food system facilitated by detoxing with her raw vegan diet, but that there can be holes for many people in vegan nutrition when they continue on long term and you worry that her current state of mental health raw food is a result great for "cleansing" the system, but what she is suffering from now is not something she needs to detox from, but an illness of a nutritional gap somewheredeficiency, her brain is starving and needs the most bio-identical building blocks possible to rebuild it, i.e. animal fat and protein.
The desire of not wanting to do harm needs to take into account the self. Denying herself what she needs to be healthy is also anti-feminist, historically veganism has been used to control women's sexuality and to keep them "in their place". In the Victorian Era there was something called "green sickness" (anemia) because it was fashionable for women to eat only salads and sweets, her diet sounds like a modern version of that. She can choose to self-select herself out of existence, or she can come to terms with where she is now, and work towards improving the future of food. Weston A. Price set out in search of a healthy vegetarian hunter-gatherer society to share with the world as an ideal, but every healthy indigenous society he encountered included some animal products.
This will take an enormous act of courage, but you may be her key to returning to health. I'd be pretty upset if I knew someone was holding out on me if they had critical information that would help me heal. There will be an initial angry emphatic "no!", but you have at least offered your knowledge and caring, and it might sink in eventually. Try to be as well researched and diplomatic as possible, and make sure you prepare to accept her unconditionally even if she refuses every single piece of info you share with her.