Children with emetophobia (or any other phobia for that matter) often ask their parent(s) for reassurance that they won't vomit, or won't see a spider, etc. I've talked with some pretty exhausted parents who day in and day out are peppered with questions from their anxious child.
In seeking reassurance you may calm down a bit if someone tells you that you're not going to be sick and it's just gas, or just some discomfort, or you're just full, etc. etc. The problem comes in when a) you don't really believe them - because after all, they're not you, and how could they know what your stomach is going to do? and b) them reassuring you that you won't be sick is like them saying that if you were to vomit you would somehow be in some kind of danger or harm's way, when in reality vomiting is not dangerous or harmful at all.
So if you're emetophobic, try to think clearly when you are tempted to ask for reassurance. Practice breathing more slowly and evenly, and relaxing all your muscles from head to toe. Say to yourself, "vomiting is not dangerous or harmful in any way, so I don't need to be afraid of it." If you're the emetophobic's loved one do not give them reassurance. It's tempting to do so because it may calm them down, but it will make their phobia worse in the long run because it's a way of avoiding what they're afraid of. Instead, help your loved one out by helping them to get professional help for their phobia. If you can't afford that, check out the "Resources" page on this website's menu and then click "Self-Help." Meanwhile, calmly say to them something like this: "I don't know if you're sick or not, but I do know that even if you are, you will be ok. Vomiting isn't harmful, it just feels like it is. I will be right here with you, no matter what.