It could be a number of things, tell her to try and keep the area clean and no make up. Something may have irritated the eye such as a lash or any other foreign object. I wouldn't mention allergies to any product at this stage as this may frighten the client and they may run with this when it could be something completely different. Did you fill in a consultation form and carry out a patch test?
always keep your insurance company informed so that they can guide you. Ask her to pop in and see you or send pictures if this isn't possible. Hope this helps, Don't worry x