Get a grip. The fact that a seventeen year old was in possession of cocaine is a criminal offense in Canada. The fact that a seventeen year old obtained an AK, or any firearm, without going through his parents, is a criminal offense.
A person afflicted with Bi-Polar disorder in America must reveal it on the Form 4473, or is guilty of a felony. Just how many felonies does one get a pass for in your world?
The mother did what she should have. The boy was OBVIOUSLY out-of-control. I own several AK-47 rifles, and it takes a special set of circumstances for me to leave one loaded, and with a spare loaded clip, on the dresser in my room. I'll have to guess that the majority of other people here are the same. Bi-Polar disorder, especially when it's evidenced with volent behavior during reorientation, is nothing to fool with. We have a micro-view of this child's life, and absolutely zero real history here. Certainly not enough to sustain the "victimless crimes" statement of yours, sir.
Bi-polar disorder has a chemical component that is fairly easily diagnosed, so it's not likely to be missed, or "thought into being". The addition of hallucinatory chemicals, or mood-altering drugs of unknown strength or composition, can cause violent behavior of the most murderous kind.
Like it or not, the young man was a danger to himself, and others. The addition of street-drugs made it even worse. The signs of leaving the loaded weapon so accessible are also a warning. While you might ignore them, the safest course of action was to remove them, and him, from normal social involvement. In today's enlightened world, that means to obtain help from the authorities. After all, if she had kept the weapon, and it been discovered by the authorities, then she'd have been arrested and charged. That would have helped the situation how?