I'm not an attorney and it's best if one chimes in here, but to this layperson not calling 911 would seem to be a link in the chain of negligence related to and following the unlawful act of furnishing alcohol to a minor and therefore part of the Involuntary manslaughter charge. Again, it's best for an attorney to clarify. It surely will be part of the Wrongful Death claim forthcoming in the civil suit.
The clearly "unlawful act" is furnishing him (a minor) alcohol "recklessly or negligently" (i.e. the gauntlet), which is the ultimate action that led to his death. Giving him that alcohol made him "stuporously" drunk (LINK: from the med examiner, see Par. 290-310) >>>him being drunk led him to falling down the stairs>>>>him falling down the stairs lead to his traumatic brain injury>>>the TBI killed him.