How soon did the rash develop? If it was within minutes of ingesting the eggplant, I would be concerned and be watching her VERY closely for any signs like hives or trouble breathing. If it was within an hour or more, it's probably a delayed-reaction allergy/intolerance. In that case, you just need to avoid the food, but no need to rush to the doctor or anything. Still good to keep a close eye overnight though to make sure no other reactions develop.
Mom to DD1 (10/07) and DD2 (3/11) I blog about our life with food allergies and eosinophilic disorders.