i would push her to tell you what exactly the "allergy" is as far as ingredients go and how severe the allergy is. It really seems like she is using the word "allergy" to mean preference (something that is really annoying to moms like me who actually have to deal with real allergies with their children). As in "I'm allergic to doing dishes."
If there is a true allergy, then you need to know what it is so that you can review all of your ingredients in your recipes, and see if you should even take the order. For example there could be an issue with cross contamination with your equipment if the allergy is to nuts and you routinely bake with nuts on the same equipment. When dealing with allergy sensitive clients, you also need to have a signed contract alerting them to the fact that while best practices are used to avoid cross contamination, that you there may be trace allergens present due to supplier manufacturing and use in your kitchen.
Also, if you have professional standards that won't let you in good conscience present a cake with your name on it to the public that is covered in can frosting, then don't. I can't think of one (reasonable) situation where I would agree to go against my signature recipes and serve what I feel is an inferior product just to make a sale. What happens if someone at that party thinks that you are the cake decorator who just uses can frosting? That would could kill future orders. Just some food for thought there on how to protect your image.