As TS&M said, this is between your buddy and his cardiologist, many of whom are divers. I know we would ask him if he had any contraindications to diving, and if so, he would need a specifically worded release from his doctor for diving. We aren't doctors and we don't determine what are contraindications to diving, and with HIPAA laws, we aren't interested in what someone's particular contraindications are.
I know none of this helps you with your direct question, but I would advise your buddy, especially if he is a client also, to take the question up with his cardiologist rather than asking on a chat board.