I have a severe nickel allergy, exacerbated by my job,--I am a professional guitarist, using strings and frets that contain nickel!
It think that nickel allergy is very dangerous. I am not sure about all the food that contains the nickel but I know several vegetables, fruits and meat that can contain nickel. For instance you can find nickel in green, brown and white beans are high in nickel, as are kale, leeks, lentils, lettuce, spinach and beans. In fruit you can find nickel in figs, pineapple, prunes and raspberries. Meat that contains nickel are mussels and prawns. As you can see you have to avoid this in order not to have any allergic reaction. I hope that this helped you in any possible way.
All the best to you,