The most important step to take is to call 911 if you think someone is having a severe allergic reaction. Many people with very serious allergies carry an injection kit with them for such emergencies, but still, a person with such a reaction should be examined and treated by a doctor, quickly. Benedryl can also be given and can help until experienced medical personnel can address the problem.
I do want to emphasize that you would want to be very sure that the person was experiencing an allergic reaction. If you are with someone you know, who has a serious allergy, you will probably be aware of that and know the signs. If you are with someone whom you do not know, trying to help that person in any way could be a risky thing to do.
I have provided a link for you that describes the symptoms of the most severe kind of allergic reaction, anaphylaxis, so you can see why it is always imperative that you seek medical attention.