No, she won't necessarily ruin her health, veganism can be a perfectly healthy diet when done with proper variety. However, in this case, she's clearly and idiot and probably will get malnourished. Thankfully, plants lack the capacity to feel pain, and even if they did feel pain it would be in a lesser capacity than mammals, so it would be a 'lesser of two evils' situation.

How do you know that animals have a subjective experience? How can an animal feel pain without a mind to feel it?

Lock yourself in a small room with a cat. Poke the cat with a pointy object. When it has scratched your face off, do come back and tell me the cat was not experiencing pain, and therefore reacted not one smidgen. Pain is experienced by animals. They have the mind to experience it. If they didn't, when my cat walked across the still-hot cooker top (they were not allowed on the work-tops, she was persistent and irritating and she did it when i wasn't looking) she would not have reacted by jumping to the floor and yowling in pain. She also wouldn't have never, ever jumped on the counter ever again. Not only did she feel the pain, she remembered the pain, remembered what caused the pain, and remembered not to do it again. It's scientifically accepted that say, pigs, are smarter than dogs and cats, if a cat feels and remembers pain, your bacon did too.