YTA for yelling because there's no reason to YELL here no matter how upset you are about it. You need to find a way to communicate better so she understand and respects you. Without yelling!

She gets mad and says the garlic needs to cook with the food and you can't just sprinkle garlic powder ontop, that it would taste gross

This is correct it's not the same at all. She needs to learn to cook without garlic. Maybe find her a bunch of articles like this which should be easy since many people are allergic. Maybe you can find a way for her to see it as a challenge to make cooking more interesting?