I'm with Peter. If there is any known condition which could lead to a diver being incapacitated during the dive, I want to know about it, and preferably long before I'm sitting in the water going through the dive plan. Making a dive with a buddy with a medical condition is a risk assessment, just as much as the depth, length or environmental considerations of the dive are.
I tell my potential buddies about ANYTHING that might make a dive other than utterly routine -- for example, the fact that I have balance issues getting out of the water, and often need a quick hand for stability. I can't imagine someone not disclosing issues like heart rhythm problems, diabetes, seizure history, or anything else that could result in a big problem underwater. Although not every buddy will make his decision on a rational basis, it is still his right to MAKE that decision.