My friend had what seemed to be a seizure while we were smoking marijuana in it. Why?

What else is in it? Seizures are not usual with smoking marijuana . . . . But who knows what's in your supply? Could be laced with other substances, or even have pesticides in it to which your friend is sensitive. Is the high from smoking really worth the risk to health?
Seems odd. It depends on what you mean by a seizure. I would want to know, if any other substances are involved. Other than perhaps having some sort of spell from trying to hole in the smoke, i cannot think of what this might be, nor have i ever heard of cannabis inducing a full-blown grand mal seizure. I would say, your friend might want to stop cannabis use.
Toxic effect. A seizure event is the reaction of the brain to a toxin or other insult. Some people are more prone to a seizure due to a toxic state that may not cause any reaction is someone else. Your friend may be especially sensitive to toxins, and he must be cautious about any drug. He may need to be evaluated by a neurologic specialist.