I would not get involved as long as your own child was not invited to the party. The parent who knows the information about the party from her son is the one responsible for notifying the boys parents. You are a 3rd party to the matter-so I would not say a word. I would also be afraid that if I told on the boy-he would seek retaliation against my son. The kid had to get a fake ID or book the air BnB in another nme. Then there is the question of where the 16 year old got that money. As long as your own child is not directly involved-it may be best to keep it out it as this can get very complicated.