I think I'm a terrible sceptic when it comes to these diets. I always wonder what out ancestors did. Did people die because they couldn't eat gluten or stone fruits? I don't know.
This.... she would have died. End of story.Go back further, and they seldom survived long enough.
She used to be able to eat normally, but things changed in her life and she suddenly got very ill, seriously ill. Without treatment she would have died. Go back far enough, back 100 years and she would have died because what she found she could eat simply didn't exist. 200 years and she would probably have died from the original cause, though that cause would not have existed then, something else probably would have killed her before she got to the age she is now (approach early retirement).
And perhaps you have the wrong end of the stick wrt to the word 'diet'. Her aim is not to lose weight. It never was. 'Diet' in this sense is simple what she can eat, not a calorie controlled weight loss program.
My 'diet' is dairy free. Anaphylactic shock would most likely have killed me, that is assuming they managed to keep me alive at birth. I spent 2 weeks in an incubator and if that had not killed me, my first major asthma attack at school (had I have been there that is) would have killed me, assuming anyone of the numerous chest infections in my childhood (cause by parents' smoking) hadn't already killed me. People simply didn't live through conditions like this, and they were never recorded because, well it just wasn't for a long time, and then it would have been put down to something else...