The PSU may not be able to provide enough +12V amperage (even though that GPU doesn't need too much). What brand and model, not just wattage, is his PSU? A quality modern PSU has full range active PFC (no little voltage switch) and 80+ certification for efficiency. If his PSU is just some cheapie that came in a case, it probably isn't up to the job, regardless of how many watts the label claims it can produce. Seasonic, Antec, Corsair, and Enermax are among the quality brands in the 430W-550W range you would want to consider.