With damaged allen heads on the fan bolts, I've had good luck tapping a torx bit into the hole. Depending on how chewed up it is, you can use a T45, T47, or T50. The radiator will need to be removed usually to be able to tap it in place, but it beats drilling, as the condensor needs to be removed then.
Just as a side-note though, I hope the original poster and his mechanic realize that the pulley hub does need to be locked into place with a special tool to be able to remove the bolt. If he doesn't realize this, maybe he thinks the THREADS are stripped and not the head?