As an ex-accident investigator RotorSwede, my opinion is that your friend talks rubbish. Helmets have been shown over and over again to be very effective in reducing the chances of fatal head injuries in a helicopter accident. The US Army has researched this to death and the USAARL website is a good source of information. Now there are helmets that have been designed for helicopters (such as Alpha and Gentex) and others that are adaptations of jet helmets. The helmets designed for helicopter use have good side impact protection and noise attentuation characteristics.
Whether to wear one or not. Well, I believe anyone who regularly flies in helicopters should wear one. There may be a risk case for occassional fliers not to.
But I have seen accidents where the wearing of a helmet would have either saved a life or at least likely negated the likelihood of brain injury.