Dude, assuming your friend wasn't 35 or older, and didn't have a congenital heart defect (which would have been the sole cause of his heart attack), then your friend dying of a heart attack because of stress is beyond the realm of unlikely.
Now, given, my grandfather had his first heart attack at age 28, but that was because he was built like an ox, was a two-pack-a-day smoker and farmer, and had just gotten back from WWII (aka: extreme stress plus arteries like sausages).
So, as far as I understand, primary causes of heart attacks in those under 35 are: drugs, alcohol abuse, diabetes, physical trauma, morbid obesity. Stress in those situations would be an extremely minor factor.
This is beside the point. You can defend your friend Gary. I'll defend any gay person's right to even marry whoever they want to. I won't defend someone's right to be overly sensitive about every remark out there that is potentially offensive to the class or type of people that included your friend.
Who does that? "YOU MADE FUN OF CAPRICORNS. My dead grandmother was a Capricorn, and died because people made fun of her for being born in late December."