The trouble with that Jeremy is that often the health professionals don't understand this way of eating at all, so not many are likely to recommend it.
Eating as stage 3 will actually give you all the nutrients you need which is why i suggested a thorough look at what she is going to eat. By all means she can seek advice first, but might not get the go-ahead she wants.
This kind of diet provides with the necessary minerals, vitamins, carbs and proteins/amino acids, just in a healthier form than conventional hamburger, ketchup, fries and soda diet. my opinion again, I'd say these are the kinds of food to avoid in general, not the 'cavemen' diet.
I just did a medical exam for my new job, the first one after I started the cavemen diet in March this year. From being constantly anemic with hemoglobin levels 105 g or less/l my current hemoglobin level is 131 g/l which is physiologically healthy and I also feel so. Cholesterol levels are ok, liver enzymes are ok, complete blood count ok, urine test ok. Re-calcification of teeth happening, mineral levels ok. If you are physically active add some honey, extra fruits or rice until you adapt to the diet or for as long as you want. Or for ever if this suits you. In a while you'll tailor the diet to yourself anyhow and you'll be doing fine on it.