If it had been an MI, they wouldn’t discharge him the next day. If it had been an MI, they would be doing extensive measures such as angioplasty, a cardiac catheterization, or other invasive procedures such as surgery if he had a blown artery or aorta or shown to have clogged arteries.
Also, it sounds as if the pain may be a reaction to medication he was taking. (Just a guess may be reading too much into what is in the note.)
So the good news is he is alive, and whatever has taken place seems to possibly be non-cardiac in nature.
They'll be more depressed than ever.