If all that can be shown is that junior is holding hands and chatting with the 19 year old girl, then this is not a court issue. No restraining order, no criminal charges, no heavy hand of the law will come down to smite the girl.
The parents can discipline junior, they can rant and rave about respect to the girl, and they can even bluff and bluster. But, unless it can be shown that the girl is somehow putting junior at risk or encouraging him to become delinquent in some tangible way, a legal solution is not what fits here.
I had an issue of an 18 year old girl taking my then 16 year old son to a party where there was alcohol, and he got caught drinking by my compadres. She and I had a sit down. I explained that I did not expect her to be his guardian nor did I expect her to snitch on him. I DID, however, expect her to respect my rules and my wishes and not to transport my son to a place where she knew there would be drinking. After a few tears and an apology, she agreed, and we've been fine ever since. She is still a friend of my son a year and a half later, she is still welcome here, and she has adhered to my wishes going so far as to bring him home when alcohol arrived at one location. No courts, no restraining orders, no threats of arrest.
Sometimes a good heart to heart beats all the court orders in the world.