RE: which one?
I too have the same problem only mine came from 23 years on Aircraft carriers and Nav Air Bases. I guess shooting and loud music played a part too. I ring loud all the time sometimes the ringing drowns out people's voices. I talked to my Dr. and he told me when I shoot from now on wear ear plugs and ear muffs this eleminates almost all sound. Tell your friend to talk to his Dr. about this. It may save him from not being able to shoot anymore. Remember this is a comulitive problem and one shot at a time it will take its toll. you could be fine but that next shot could be the one to put you over.