I know at least 2 drivers with pacemakers fitted although they were fitted after they began driving. As long as your friend can demonstrate that there is no more risk of them keeling over than anybody else there shouldn't be a problem. I would be prepared for a visit to a cardiologist though and if you get wind that you have a medical for a job then it might be an idea to pre-empt this and ask to see one first (NHS or private, not the TOC!) and get a letter to take with you that says you are perfectly healthy. It would be difficult for an Occupational Health Doctor (who, with the greatest of respect, are notorious for misreading ECGs) to justify failing your friend, overrulling a professional cardiologist in the process. In fact it would be utterly absurd.