Overclocking by someone who is inexperienced can be very harmful, yes.
But if someone knows what they're doing, doesnt push too much at once, monitors temps and does stress testing, they can overclock without damaging their equipment
There's a wealth of information about how to overclock safely on this forum and on many, many others
IE depends on what your friend was doing, was he trying to double the frequency of his CPU? Push the PCI bus past what it can do? etc..