This might sound a bit odd, but I plan on telling my (coeliac) children something which I have gradually realised during the course of living life with a non-food-related disability. When people say or do unpleasant things in reaction to your disability, then your disability has done you a service by revealing that person's true personality. That's a handy shortcut when getting to know people.
Also, I wouldn't advise your DC to engage with the specifics of what the person has said. I'd suggest something along the lines of 'What a nasty thing to say, why would you say something so bitchy/mean.' Or 'Why are you so interested in what other people eat?' or 'What's your problem?' Pointing out the unpleasantness in the other person, rather than defending her diet.
And also (which again, I am trying to do with my DC, who are much younger so bullying isn't an issue yet), I would keep reinforcing to your DC how brilliantly the diet is working for her, how fantastic it is that all it takes for her to be well is to keep to the diet' etc. etc. Keep building up and stressing the positives of the diet - the difference it is making to her body.
And finally, I plan on saying to my DC, when they are approaching the teenage years and will have to take more control of their own diet - 'It's up to you, you have the choice about giving your body what it needs to be healthy and strong, or you can hurt your body by giving it damaging food. Sometimes that decision will feel tough, but I know what a strong, determined person you are, and I believe that you can make the tough decision when it counts.' So, facing up to / putting up with other people's negative reactions will be part of what can make the 'right' decision tough, but I want to make my DC feel that by doing that, they are proving their own strength and capability - seeing the bad experience as 'character building', I suppose.
Good luck to your DD. I thought DC1 had CVS for a while, but it turned otu to be coeliac. I read up on it though, so I know a bit about how touch it can be.