How to help someone who is having heart attack instantly?

FIRST 911. Before you do anything else, get help on the way. If their heart attack leads to a rhythm problem with the heart, you want a defebrillator there. Lie person down, loosen clothing and have them chew an Aspirin - it absorbs quicker that way. Stay calm and make note of any specific symptoms they describe.
Call an ambulance. Have the person rest, take deep breaths, chew a baby Aspirin or swallow it and call an ambulance right away or take him or her to an er as soon as possible.