If indeed the rest was minimal and/or if the weights were such that the old man, whom we will assume was unaccustomed to any recent difficult exercise, was having to strain to complete the reps, then this was stupid and I agree could have contributed to the problem.
By the way, I have no idea whatsoever why I previously had the idea you had said he was on a treadmill at the time of the seizure. You did not. My bad.
Also not being a professional in that kind of thing, I don't know, but tend to think the cause, if exercise releated, was probably was more degree of nervous excitation in the brain rather than heart rate per se.
Well, increasing strength is very important and helpful for the elderly but as you say the protocol should not be one driving the heart rate up unduly, and also -- for other reasons as well -- should not be so demanding on the nervous system. Among the other reasons is that explosiveness and speed put unnecessary risk to their joints and bones as well as cause unnecessarily greater risk of muscle injury. Slow and highly controlled movement is the way to go.
Which these kids would not, I assume, have done.
At this point it seems only a guess that the kids pushed him after being woozy or having sporadic, jerky movements, etc. Even if stupid, it seems easily possible, even likely, that they weren't THAT stupid.
A whole lot of seizures not even an MD present on the spot has any idea is going to happen before it does.
It just seems to me a very high chance or even predominant likelihood that while the kids may be stupid and lousy trainers in general and even worse for the elderly, that this event was unrelated to their actions, unless they were pushing the old man "high intensity" style, in which case I guess it's a quite reasonable likelihood.
Just as pure shooting-the-breeze guesswork: the answer seems impossible to know either way at least with the information available. But the general question of whether stupid/ignorant trainers are likely to cause seizures in the elderly is an interesting one that seems important.