I'm a cardiologist myself with an interest in diving medicine. According to "Medical Examination of Sport Scuba Divers" by Alfred Bove, M.D., Ph.D. (third edition), "anticoagulation used with mechanical valves can increase risk of bleeding and patients must be so advised. The presence of a mechanical valve per se with normal cardiac function is not a contraindication."
That having been said, the most important concern is what condition did your brother have that resulted in a mechanical heart valve. If he had a bicuspid aortic valve with normal heart muscle function that required a valve replacement and he has normal exercise tolerance there is really no concern with diving other than the coumadin issue above. However, if he had a "leaky" aortic or mitral valve that resulted in a weakened heart prior to or as a result of the surgery, he should probably undergo exercise testing to assess his functional status before diving. PM me with details of his condition if you have any questions.