This question involves hypersensitivity and requires that you know how a hypersensitivity reaction occurs.
Question A: He is wrong because the woman is not suffering from a clogged airway, which could easily be cleared with the Heimlich maneuver. She is actually having a hypersensitivity reaction to something she is allergic to.
Question B: It is possible she is allergic to peanuts since she questioned the waiter about them. Many people are allergic to peanuts and will have a severe reaction to them.
Question C: The best way to prove what happened would be to ask the women if she was allergic to peanuts and then find out if there were any peanut products used in preparing the food she ate.
To answer this question you have to consider autoimmunity.
Question A: His problem is caused by an autoimmune disease in which his immune response has begun to react against and destroy the islet cells in his pancreas which affects the production of insulin and causes the disease known as type I diabetes. Unfortunately, the cause for this reaction against self-components is not understood.
Question B: You can explain to him that the immune response is actually the reason that we survive in a world filled with dangerous organisms that can hurt us. This system of protection is extremely specific and normally does not turn on the host but in some as yet not understood circumstances an autoimmune response can occur. Sadly, this is the situation he now faces.