If that is correct then I would think that the cashier and his supervisor are going to be onto at least a reprimand from Asda for their behaviour, but hopefully more than that, as they have engendered some very bad publicity for the store.

If I had been the customer I would have insisted on talking to the shop manager, having taken note of the identity of both the cashier and the supervisor. these people are paid to serve the public and should not be making decisions about who can and can't be allowed to purchase goods in a store that does not belong to them. What would happen if this woman was taken to hospital and a nurse or doctor refused to treat her because she was wearing this wristband? Having strong views about things is fine, just leave them at the door when you go to work.