Hi T.,
This is a tough one. I have had a vomit phobia myself since I was a very small child. It is difficult to completely erase (I still have it) but it can be managed. Vomit phobia is officially known as emetophobia and you can find a therapist who has experience with treating it at www.emetophobia.org.
I have also used a lot of self-hypnosis and guided meditation CD's to help me relax. Relaxation is critical - you can't be afraid and relaxed at the same time. Once you teach the brain to relax it helps let go of the fear a bit. On the other hand, thinking the thoughts over and over again programs them more deeply into the brain. Two CD's I recommend are Relieve Anxiety by Dr. Steven Gurgevich and Fear of Vomiting Self Hypnosis (available at http://www.hypnosisdownloads.com/fears-phobias/emetophobia).
I agree with the suggestion to look at diet; however, I actually am a nutrition consultant and obviously it hasn't solved my problem (much as I love nutrition, phobias are generally very complex). There is no magic bullet solution. I would definitely eliminate sugar, trans fats and processed foods. Getting adequate protein is key because the brain's neurotransmitters are made from protein and in most phobias the brain chemistry is out of whack in some way. A wonderful book that looks at anxiety and phobias from a holistic perspective (and gives tons of nutritional suggestions) is The Chemistry of Calm by Henry Emmons.
As far as supporting her, try to find out what the real specific fear is and reassure her that you will be there with her every step of the way. This phobia is a very lonely one and knowing she's got you on her side will really help. Don't try to tell her the fear is irrational. She knows that. It's hard to describe if you haven't lived it, but the fear itself feels very uncontrollable and for me, it's something I'm ashamed of and so don't share with anyone other than my husband and therapist.
Good luck to you and if you find a cure, please let me know! I wrestle with this every day of my life. I try to keep a sense of humor about it; some days it's easier than others.