Actually, eggplant allergy is not all that uncommon (same goes for mango, FYI). Any other symptoms or just rash around the mouth? If only the rash and if it goes away quickly I might still give it but just not too often and keep an eye on things. However, I believe eggplant is a member of the nightshade family - which also includes peppers, tomatoes and potatoes. Does she react to any of those as well?