If they're standard H bulbs, the most common "blinky/dim" problem is a burnt ground pin on the headlight connector. Sometimes they actually get hot enough to melt the plastic. For that reason, almost all parts stores sell pigtails. If he has LED aftermarkets, chances are the bulb is failing. If he has aftermarket HID's, it could be anything south of the power connection, ballast, igniter or bulb.