The itching could definitely be linked to her diet. But only switching out a meal or two isn't going to help her issue. She would need to go all in and completely remove the bad foods from her diet for several weeks before she will notice the difference.
My guess would be gluten for causing the itching but I've also experienced itching as an issue when eating dairy.
If you can convince her to just give it a try for awhile, it will make a difference. Cutting out all gluten (grains), dairy, legumes and processed sugars for a few weeks will put her on the right track.
Its not an easy change for the carb addicted but giving up a few foods seems like a better alternative than going crazy with unseen itches.