Tell your buddy to forget about doing something illegal and think about this.
One gunshot registers about 145 decibles. Noise-induced hearing loss is usually permanent and progresses with each exposure so, while a single gunshot does irrparable damage, so does prolonged listening to rock music at high volumes in the car at 115-120 decibles. Hearing loss is like back injury, it's cumulative and will get progressivly worse if you do nothing to prevent it, we all have some loss either from our work environment or any number of Aerosmith, Pink Floyd, Lynard Skynard and dozens of other concerts I've seen since 1973...
Don't worry about it!
Tim