My friend is scared. He will bite at anyone who coem near his mouth. He's fifteen for crying out loud! Look, he's scared and I have done my best to explain to him they won't hurt him. Can I have some more advice to give him?;)

Does he have a mental handicap? At fifteen that sounds childish of some sort. In fact he should be afraid not to, since certain teeth issues can cause seroius health problems

Fr_Chuck is right. If teeth with cavities are left untreated they can become infected which can run through his body and cause serious health problems including death.
Suggest he see a dentist who can sedate him either with valium or laughing gas.