You don't know these people well enough to even know what state they are in (you don't know if they are really boyfriend-girlfriend, if they have even met, let alone if they are actually having sex, i.e., doing something illegal). The girl "pretty much" sees the guy as a boyfriend, so if they are doing something illegal, it is not forcible. If they are doing something illegal, it's not going to be illegal for much longer since she will be 18 soon enough.
So why do you want to get involved??? Leave them alone.