Hi Mara,
I was like your daughter when I was young, I liked only vegs, and disliked all meat, practically turned into a vegetarian - back then, no one talked about vegetarian, and few people were. However, occasionally I had instinctive feeling that I was lacking nutrients if I eat only veg., so I kept myself on little meat all the time. After giving birth to my son, my stomach turned havoc on me, so bad that I was disappearing and unable to work. I started researching for a cure, and here is what I have found and they seem to help a lot for both me and my son: 1) There are some nutrients only available in meat, eggs, diary, not in simple vegetarian diet - you may have to really go extra miles to get such nutrients in vegetarian diet. 2) Stop diary for 2 weeks or more to see if her stomach is more consistent. More people have problems with diary products than we think. After that, try only goat milk based diary products, kefir and perhaps a little raw milk cheese - these are much easier to digest than other ones. Though in my case, I can't handle any of them. 3) Beans in veg. diet also give many people stomach problems; besides most beans have enzyme prohibitor (which prevents beans from sprouting), eating too much of these can cause one's own enzyme to not work effectively to break down food for digestion and obsorption; 4) Omega-3 organic eggs are excellent source of nutrients if she eats eggs. 5) Some raw vegs. are also difficult to digest - I know everyone seems to be thinking raw food is good, but after couple of years of experience with raw veg. juicing at home, I admitted at last, for me, well cooked vegs. are much easier on my stomach. 6) If possible, stay away from wheat and wheat product. Gluten in wheat is very hard to digest and can disrupt digestion for other food. WholeFoods has rice based bread, it has worked great for me and my son. 7) Other "healthy" grains such as the commercially advertised heavily Oats is a big problem for me for a long time that I did not know - thinking I was eating healthy, only when I stopped it I felt so much better with gas and bloating, etc.
My son used to have stomach "bugs" a lot too, always seems to be fluctuating like mine. When we made the above changes, he seems to be handling everything well and the stomach bugs are gone for good.
But each individual is different. She may do well on stuff others don't, or vice versa. So perhaps the best way to figure it out is this:
1) Keep a food diary, note down what's eaten, and how she felt.
2) Keep suspected food out of diet out for 2 weeks, and see if there is a change. If you are not sure, add that food back, and you should notice stomach issue within a week when you add it back.
3) Start eating simple food, and add one extra type of food into diet at time to check if the system is handling well.
4) Homemade smoothies. I have got Vita-Mix, and it's great to make smoothie - you can put whole veg and fruits in with all the fiber and everything, it comes out completely smoooth and helps digestion great deal.
Good luck,
W.