Hello, Welcome to Just Answer and thanks for your question. Diarrhea is common in viral gastrointestinal infections. They can cause sweating, nausea, and cramping as well. With a normal ecg and heart enzymes, it doesn't sound like this episode was in any way cardiac-related. Keeping him overnight and doing serial enzymes is standard procedure with someone who has a history of ischemic heart disease, but it sounds like that wasn't the problem this time.
Sorry, I had to get some sleep, too, and I just signed back on again. It is 11 a.m. on the east coast of the US and I have had back-to-back patients fail to show up for appointments, so I have a minute to catch up on Just Answer before the next one arrives. It is always scary when a family member is sick, even when you are a health care professional. It sounds like your father overdid outside, perhaps having a bit of heat exhaustion, if not heat stroke, which precipitated his symptoms. I'm glad he is doing better and it wasn't cardiac. God bless you all.