Serotonin syndrome is quite unpleasant, but rarely fatal. Moreover, a dose of mushrooms would be unlikely to result in seratonin syndrome, regardless of the size of the dose. I'm not saying its impossible. Just highly unlikely.
The most likely explanation is an allergic reaction to one of the other chemicals in the mushroom. Anaphylactic shock seems the only real possibility to me, in light of the fact that I have taken half an ounce of shrooms all at once on many occasions. Dried cubensas each time.
Surprisingly, my hardest trip (which is to say, the one that seemed the most physically dangerous and frightening) was on only five grams, dried, eaten all at once. This is actually less than my usual standard dose, which is a quarter ounce.
I have also discovered that even at fifteen grams, the duration of the trip is still rarely over eight hours. I've managed to get over twelve hours, but I had to repeat dose several times and the intensity of the trip was diminished significantly as the physical discomfort factor rose.
I want to stress that I feel a certain horrific vacuum in my spirit right now, reading about this event. For some reason it hits me real close to where my heart is, and I can't quite pin down why. I think it is connected to whatever caused me to become frightened of LSD and stop taking it, even though I have taken 1500 mics at once and tripped for over thirty hours and still went back for more over the next few years. I found the LSD threshold dose for me to be about 700 -- 800 mics. Below that, LSD is more or less a mental slide show, and while certainly entertaining, the full scope of what LSD has to offer is lost on me. Of course, everyone is different, so these doses may have dramatically different impacts on some of you.
However, death seems a long shot unless by an allergic reaction resulting in respiratory failure.
There is one other possibility. If the child had a pre-existing kidney condition that was undetected, certain adulterants and chemicals commonly found in mushrooms could cause renal failure.
Regardless of the actual pathology, the event is tragic in every sense of the word, and the family and friends have my most heartfelt condolences.
To the individual that knew the child personally:
A special prayer for you, that you may find peace and closure. Also, a prayer that you be given words with which to impart comfort to his mother, for the unexpected loss of a child is the most tragic and unfair scenarios. Where does one find the strength to go on from there? I believe strongly that we are watched by angels, and that an angel of peace will come to her aid for the asking, should the request be made in a spirit of humility and honesty.
Perhaps the boy's spirit was needed elsewhere to save another? Is it possible that this tragedy is in fact a spiritual transplanting that saved the soul of another? What a blessing that would be! To discover that what once seemed senseless was by design of a divine nature...and what reward awaits the grieving mother? Truly I pray that she find a joy that surpasses all understanding, a joy that comes from the Holy Spirit and is dealt personally to those in such need.
My faith is weak, my heart is cluttered, but I will pray for her anyway, because I believe that it is by grace our prayers are heard, and not by judgement.
I know this first hand, because I live and breathe by the miracle of grace that granted life from a prayer.
Peace to you who have been wounded by this, peace from the Almighty, from whom all good things come.
although of course, individual idiosyncrasies still apply greatly.