"And then you have people who suffer from mental/medical condition, it's very rare that this happens to a clinically sane person with no brain defaults. Suicide is never an option."

Exactly why it's not the boys fault. If she was severely depressed I doubt there was anything he could have said or done to stop her. Perhaps he could have agreed to marry her and that would have prevented her for a short time, but eventually the depression would come back and take over.