I have not tried the specific diet in the book, but I have tried modifying my diet many times in many ways but with no success. Food (other than alcohol) is not really a big culprit in triggering my migraines. Mine are usually related to hormones, stress/excitement, and sensory overload--most often a combination of those. Obviously, DL is a place that can be hard on a migraine sufferer.
I have had horrible migraines since I was 6. They've changed over the years but unfortunately, I still get them frequently. My neurologist even enrolled me in a Botox study but that didn't work for me. Imitrex injections are the only things that work for me consistently, and I think of them as miracle drugs. I think back to the old days before Imitrex with a shudder!
I have tried Beta blockers and other preventive prescriptions (Elavil and I forget what else). I do not seem to tolerate medications well and had very bad reactions to all of them. In addition, I hate taking medication every day that totally changes my body chemistry in weird ways.
About 6 months ago, I began having rebound migraines. I'd end up using the injections for days in a row. So, my doctor started me on...hmmm, what was the name of it? It's a seizure medication...I can't think of the name of it at the moment, but it seemed to work pretty well. However, I really didn't like the idea of altering my brain chemistry and had avoided trying this medication for that reason. There is one that does the same thing called Topamax. Anyway, I began to notice that I was gaining weight. After 3 months I was 15 lbs heavier and not eating more so I began to realize that it might be the medication. Did some research, asked my doctor, and sure enough--it was the meds! The average person gains 60 lbs in a year on it! Sorry, but I'll endure the headaches rather than gain a bunch of weight. It also made me foggy, but Topamax is supposed to be even worse (it's nicknamed Dopamax). The advantage to Topamax is that its side effect is weight loss rather than weight gain, so I could put up with fogginess for that

. Unfortunately for me, I can't take Topamax because it also triggers kidney stones and since I'm a chronic stone maker, my doc said no.
So....we looked into it, and guess what? My rebound headaches started after Imitrex lost their patent or whatever it's called and pharmacies began offering a generic version. Turns out my insurance company tells the pharmacy they must use generic unless specifically advised not to by the physician, so I'd been using generic. Well guess what? The generic doesn't work for me like the real McCoy! It takes away the headache for a few hours and then I get it back, and back, and back, in a vicious cycle. Maybe your sister has run into that same problem?
I stopped the...Depakote! That's the name of the seizure med I was taking...I weaned off that and my doctor specified non-generic Imitrex and I'm back to my old pattern, which while not great is at least predictable and treatable without rebounding. Have your sister make sure she's getting name-brand Imitrex if she is having a problem with rebounding.
ETA: I worked hard at it this summer and lost the 15 lbs plus another 8 so all's well that ends well