Well, he must have cross threaded the screw or used the wrong size screw 
driver.  If it is absolutely rounded out beyond repair, it is a bit 
tricky.  I'd try different size screw drivers.  If all else fails, get a 
lift handed or backwards drill bit.  Yep, the make them and I hav used 
them for just such problems.  Has to be about the right size, much smaller 
than the head.  The idea is that once the bit catch as it turns counter 
clockwise, it will catch the screw as the bit bites in and losen it.  Good 
luck.
73s
Butch Bussen
wa0vjr