Generally, cocaine seizures are self-resolving but they are still life threatening and considered a sign of severe toxicity. Only a doctor can determine what needs to be done for any given seizure and there is no rule that says, "If he doesn't swallow his tongue, turn blue, and require CPR, then its just a baby seizure and doesn't require medical intervention." Even if you recover without medical help, its still reccomended that you see a doctor immediately.
Most cocaine deaths (non-traumatic) are caused by tachydysrhythmias (arrhythmias, irregular heart beat). Unless you hook yourself up to an EKG when you do coke, you probably wouldn't know that your heart rhythym was dangerously irregular.
Some deaths are also the result of myocardial infarction (heart attack). Signs of this would be severe chest pain lasting for at least 30 minutes, shooting pains in your left arm, paleness, sweating, vomiting, breathlessness, and myriad other symptoms (but could be relatively symptomless).
Agitated delirium commonly accompanies lethal cocaine toxicity. The symptoms would be bizzare and violent behavior, such as aggression, combativeness, hyperactivity, hyperthermia, extreme paranoia, unexpected strength, incoherent shouting, and terminating with cardiorespiratory arrest.
Hyperthermia definitely contributes to fatal outcomes, and seizures can further increase temperature.
There are so many possible symptoms and complications of cocaine toxicity that only a doctor will be able to determine how serious any particular case is. And you don't have to worry about self-incrimination if you take your friend to the hospital. Unless the person has an eight ball in his pocket when he arrives, there is no danger of getting in trouble. The only reason he would then is because the doctors would probably check his pockets and maybe get an analysis of the substance. Then they would naturally call the police to confiscate it who could press charges if they wanted. But if you have no drugs on you at the hospital, nothing will happen to you, so take your damn friend there next time he has a seizure.
Much of the information in my post comes from here:
http://www.emedicine.com/emerg/topic102.htm