Is this a GT? If so he's probably running with his foglights on, thats the problem. They draw too much power, causing the thermoprotect in the switch to open, ie your lights turn off. After a while you'll get that roasted harness that bmorris post.
You need to either re-wire the circuit with a heavier guage wire, OR be like me and install relays and take the power directly off of the battery. If you use a relay, you don't have to mess with the wiring in the dash, and you can put the wiring back to stock (dunno why you'd do that, but if you want) fairly easily. I have 3 separate relay circuits for my low, high and fog lamps.