We tried seeing a therapist who kept focusing on what was the initial cause of it, which we never got to the bottom of. We had 6-7 sessions and to be honest it seemed to make her worse as it really focused her on it, and the therapist at one point said 'this is the WORST POSSIBLE PHOBIA you could have!'........
...not helpful at all.
After we decided mutually that we weren't getting anywhere, she suggested we try reading Susan Jeffers 'Feel the fear and do it anyway', which we are doing with her with some success. It's an adult book so we read it to her and relate it to her fear.
I have also discovered the Thrive program by Rob Kelly. It was actually a thread on MN that took me to it. Search him on You Tube, lots of useful videos for you as parent to watch (not suitable for the child IMO). I have also ordered DD his book, specific for youngsters 'Cure your emetophobia and thrive' by Rob Kelly. It's a work book that you work through with your child. The theme is very similar to the Feel the fear book, which I like. It's all about changing your locus of control and your mindset. The other thing I like is that both books say it doesn't matter what the initial cause was, it's irrelevant, the point is to move forward.
My DD has already made some really good improvements. Just two weeks ago she was coming down to us most evenings, inconsolable, saying all she could see and hear when she closed her eyes was people being sick.
She hasn't been down to us for over a week now. She says she feels more in control. She's not over it yet but it's a six week program. She is definitely improving and we as a family are very positive about it.
The child needs to know that it will take time and effort but they CAN recover from this.
I'm v keen to liaise with other parents going through the same. I think this Thrive program is worth a look at.