If you watch someone with asthma develop extreme difficulty and then turn blue and stop breathing, what should you do?

CPR & 911. Regardless of the cause of apnea do the following, (1) check airway for obstruction like a foreign body (2) give 2 rescue breaths if patient is not breathing (3) dial 911 (4) check pulse for circulation (5) begin chest compressions if there is no pulse and you know how (6) continue CPR until paramedics arrive & take over. Everyone should know basic CPR especially when family members are at risk.
Call for help. Patients that stop breathing with asthma are in dire circumstances and need expert resuscitation. These patients often require complex measures to support respiration and the rapid administration of drugs to open up the bronchial tubes(bronchdilators). The mainstay of initial therapy is administration of albuterol or similar drugs. If the person has an inhaler it should be used promptly.!
Call 911. This is a medical emergency requiring acute medical intervention. CPR may be required by bystanders. Make sure there is no airway obstruction in the mouth that you can see and remove. Consider giving mouth breathing and place the head and neck in a position (sniff position) to optimize breathing.
Call 911. First, be sure 911 is called. While waiting for emergency personell to arrive at the scene, you can provide rescue breaths. You can be trained in this technique by taking CPR training, if interested. If you do not have this training, the 911 operator can instruct you while waitin for help to arrive.
911. Call for emergency services immediately. Then you can start cpr. If they have a nebulizer you can give it a try.