Hi.. Welcome to HEALTHCARE MAGIC.. As per your complain sudden severe swelling on face that is resolved with steroid pack is pointing towards allergic reaction only and can be probably due to any food or medication that she has taken recently.. If you are taking Percocet since long then it does not seems to be the cause.. I would suggest you to consult an Allergist and get allergic tests like Patch test, RAST test, food challenge test etc can help in diagnosis and treatment can be done accordingly.. The tests will help in ruling out the exact cause of the allergen and once you know the cause of the allergic reaction you can avid exposure to it to prevent allergy attack.. He can be advised to take anti allergics like benadryl along with steroid oral as well as injectables.. Do cool compresses over the face and drink plenty of water.. He can also be instilled an Immunotherapy that will help in reducing the severity and frequency of allergic attack.. Once the allergen is ruled out avoiding exposure to it can prevent the allergy symptoms.. Hope this helps.. Regards..