The age of consent is 17. They raped each other and can be considered sex offenders for the rest of their life which will impact where they can live, what they can do and if they can actually parent any children in the future. The sixteen year old is most likely to be charged for a lot of public policy reasons and could spend some time in trouble. How is that for scaring them.
You however as a friend have no right to do anything. Unless you want to report it to the police and let them investigate the rape. The sex crimes unit would be your best bet.