My son (now 14) was diagnosed with CVS when he was little. We were told that he would eventually outgrow it and it would turn into migraines with potential vomiting. The doctor said to think of CVS as a migraine in the stomach since little kids rarely get migraines. Eventually the symptoms flip from the stomach to the head with periodic stomach problems. I don't believe my son has ever experienced a true migraine but he did get frequent headaches as he aged. Over the years, he seems to have outgrown them although he still can not see 3D movies and he needs Bonine for a long car ride or any plane trip.
I am sure nutrition has something to do with it but I can't pinpoint it to any one thing that worked for my son. He has always been plagued with allergies so his diet has always been pretty healthy. He did not eat any nuts, soy, eggs, dairy, fish, or beef when he was little. He also never had anything with preservatives or dyes. His CVS started at 2 and subsided for the most part by 7. His headaches intensified by 6 and subsided, except for the few things I mentioned, above when he was 10. We started eating this way when he was 12. For him, that only meant giving up pork, poultry and oil so it wasn't too hard. We had been eating things like brown rice, potatoes, and veggies all along and rarely ate out.
Although the vomiting was beyond horrible, it seems as if my son had a milder version than most kids because he outgrew it quicker than they predicted and didn't have the severe migraines that they predicted. I don't have any proof that his healthy diet was the reason for the reduced symptoms but I am pretty certain that foods full of preservatives or dairy products wouldn't have made him any better.
Best of luck to your friend. I have a major vomiting phobia so that was a fun time in our house!