Nobody's gonna help you with a heart attack or a vessel in the brain going pop at "any time". It's just that your chances proper help in time are better when you aren't under water breathing through a straw out of a tin can.
So when you filter out OW "emergencies" where you can just swim to the surface, and medical emergencies where your chances are not so good anyway, what's left?
What's left is whatever your instructor taught you to do when your buddy needs assistance. As the major example I can think of, I was taught how to donate a reg if there is an out-of-air emergency. I wasn't taught how the buddy system can help with a heart attack, so I wouldn't consider that as a factor in using the buddy system. The bottom line is that if you're diving under the buddy system, you stay however close you believe you need to be, and keep however close an eye on your buddy you believe is needed, to do the things you were trained to do for your buddy "in time," and your buddy does the same.