Legal aspect of "In loco parentis"
This is a base assumption that both victims were minors, he owned the property and was the foster parent. Depending on the aforementioned some of the following may or may not apply.
The adult was obviously on the side of the girl with the knife,he having attacked the1st victim after the initial attack by the girl with the knife. In foster care or any other situation dealing with a minor if you are the adult supervising the activity or own the property upon which an act takes place by a minor you are in part possibly responsible for the minor's actions. If he owns the property , was the foster care giver , and allowed the girl to to maintain possession of the knife on his property he is also responsible. We know the "call in" involved a knife therefore the adult too was aware of it yet did nothing. He is also possibly guilty of assault and battery on a minor as well as a possible accessory to the attempted stabbing. This frame capture leaves little doubt as to his actions .
IRONY: The adult can be heard saying "You shot her. She's just a kid man …what can't you understand?"
I wonder what was going through his mind while he was assaulting the other juvenile victim himself.