Look.
Whether sex between a 16 year old and a 19 year old is legal, illegal, harmful, not harmful, molestation, not molestation, ad infinitum is not the point; nor is it what the poster asked.
What he asked, and what is the point, is that at 16 she does not have the legal right to move out without permission and that anyone who harbors her can not only have charges placed against them but can end up in jail.
He needs to tell Little Miss to get her ass back where it belongs before his ass ends up in a very uncomfortable place.