What do you do if someone beside you develop cardiac arrest?

I would call for help and then begin treating the person IMMEDIATELY! If you have not yet done this, get CPR and first aid training. It culd save a life.

that would depend on who the victim is.. if the victim is an adult then i would first check for the victim's responsiveness, then let someone call for EMS/ambulance then do CPR. if there's AED readily available use it first after checking if the victim is not breathing and wait what the AED tells you. it would be different if the victim is a child, i will check for the victim's responsiveness and if unresponsive, i'll perform CPR immediately and use AED if available then call for EMS/ambulance. continue doing CPR and using AED(when advise to do so) until the ambulance crew arrived.

I can't receive normal cardiac-treatment due to having a connective tissue disorder that affects my heart-valves, arteries and weak connection between my chest bone and ribs making treatments such as cpr a dangerous prospect. This is why I have come to realise that I would first check the person for a Medical-Aid bracelet or any other visible indication of a medical condition before try to help them. I know that it might sound risky and a waste of time, but its the same as with a motor vehicle accident. You should never move the person even if the fuel has started to leak out. It very rarely happens that a car actually catches fire and the chances of you hurting the person by moving them is much higher than the chance that the car might catch fire. I think the point being, that if you are uncertain about how to help the person...even if you are trained in cpr...if you are not a medical professional, the best is to first phone for help and they can assist you and guide you how to help stabalise the patient while the paramedics is en-route. This obviously depends on the situation of course...but to me that's the safest bet.