There are some rare individuals whose diet needs ingredients found only in animal products. I have come across exactly 2 in my lifetime. That said, these people

There are some rare individuals whose diet needs ingredients found only in animal products. I have come across exactly 2 in my lifetime. That said, these people would still be vegan if they ate animal products. Veganism, by definition, seeks to exclude animal products "as far as is possible and practicable". I don't know enough about your father's case but this might be more co-relation than causation, based on your facts. If going vegan was causing heart issues in people, we'd really know by now.