Sporadic Choking & Coughing

My friend recently completed chemotherapy treatments for breast cancer.  She also took a drug called Herceptin.  Since her completion of treatment, she has experienced severe choking and coughing spells that come at no particular time or activity.  She feels something in her throat just before the attack and does cough up some phlegm.  The attacks are very severe, and anyone who is around when it happens worry that she is actually going to choke until she passes out.  The cough is very deep sounding.  She has been to an allergist, an ENT, and even a lung specialist.  They have done a CT scan, swallowing test, throat imaging, and lung tests.  None of the doctors seem to know what is causing her choking and coughing.  The lung specialist seems to think it is allergy related.  Has anyone else had this problem, or side effects from taking Herceptin?