People with nickel allergy have problems when the nickel contacts the skin for a prolonged period of time. The nickel in pots and pans wouldn't cause problems.
Lots of jewelry contains nickel, and it's common for people to have a reaction from earrings, necklaces and watches. I can't wear certain jeans, because I get an intensely itchy rash around my belly button from the clasp/fastener.
Flip flops wouldn't contain nickel, although some sandals might have rivets or metal decoration that would potentially have nickel in them.
Flip flops are likely to have other allergens, like latex, or chemicals used to stabilize the plastic. Or she could be having an irritant reaction, similar to a burn, (rather than allergy) from the chemicals in the flip flops.