Although Diplom might normally be translated as 'qualified', having a Diplom in psychology does not make you a qualified psychologist. Further training is required before you are qualified. Obviously in this case the person is qualified and to avoid using 'graduate psychologist', I would just say 'is a qualified clinical and health psychologist' (I think 'medical psychologist' is used in the United States, but 'health psychologist' is used in Britain and the United States.