She sounds addicted to the things she is allergic to, hence why she doesn't want to give up her junk food. A person can eat very healthily without wheat, soy and corn (I'm allergic to these 3 also, plus many more). And there are some really tasty recipes that don't require any of these 3 foods. But the easiest way to transition is to do it slowly - one or two foods at a time. Look for substitutes online and in health food stores. Take her shopping with you and ask if she would agree to try alternatives to her regular junk foods once a day or once every few days just to get her used to the flavor of the alternatives. There are some excellent pretzels out there made from rice flour, for example.
I hope this helps - good luck,
Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, T- & B-Cell Lymphopenia, malabsorption/malnutrition, Lyme Disease (Igenex Lab IgM WesternBlot positive/CDC negative), etc.; Currently TPN-dependent.
Meds: Pulmicort, Zofran, Scopalamine Patch, Injectable Vitamin B12, Heparin (to flush PICC line).