All these suggestions to call the cops are important, correct, and probably the most ethical solution.

Just sayin, though, when I was a 16 year old male, I was beggin' to be groomed by an older girl. HOLY SMOKES that would be so hot. I laid awake at night DREAMING about being sexually taken advantage of by a 21 year old girl.

When you report this to the cops, they're gonna be all "niiiiiice."

is she his teacher?

is she his mentor?

does she hold any power over him in any way?

Shes living with his parents for christs sake.

I'd cut off contact with the girl, delete facebook, hit the gym, but BUTT OUT. He'll be legal before the court proceedings get off the fucking ground.