Myocarditis, inflammation of the heart, was first recognized as a possible side effect in Israel among men under the age of 30. There were no disproportionate amounts of other populations contracting that condition, neither in Israel or in the US, where the FDA concluded that there is a rare chance of – again – men under 30 developing myocarditis. It also is extremely treatable with medication and hardly ever results in death. The signs are also very noticeable; elevated heart rate, chest pain, etc….
Did your friend have any other issues? Originally the FDA was reluctant to classify myocarditis as a side effect because the condition happens to people at a certain rate in general. We’re talking about millions of people, a percentage of which will unfortunately contract certain conditions over the course of a few weeks or months. Only if there is an elevated instance of the condition developing should there be an indication of causality, bot mere correlation. That’s a distinction that people don’t seem yo understand in the anti vaxx movement. Anecdotal evidence is not evidence at all, especially when dealing with millions of people.