The legal wording in this case is critical. The father making a firearm available to the teenager should not be the focus of the charges against the father in my opinion. The charges should center around making a firearm available to a mentally ill person. Otherwise there is the slippery slope of any parent being charged for allowing their teenager access to a firearm for legal reasons.
On the "expanded reach" portion can we then charge parents as accessories to murder when their teenager kills someone with a car that the parents bought? The base logic is there and is the same kind of logic used to sue firearms manufacturers for crimes carried out with their products. At what point do the courts hold the person who commits the crime accountable and leave it there?