Skin resistance soaking wet in a bathtub is not the same as dry skin. The piping to the tub may be metallic providing for a good ground connection to faucets and the drain.
The charger has a peak voltage of about 170 (120 x 1.414) within the wall wart, only needing an electrical fault to supply a sufficient leakage current to the phone charge connection.
You are correct that peak to peak voltage would be 120v x 1.414 (170 V) to correct from RMS voltage. I agree that there would have to be some kind of fault in the charger to let line voltage get to the cell phone, but that should have fried the phone battery and probably the phone the first time it was plugged in.
I allowed that skin resistance is markedly lower when wet, but 5 V should not be enough to be fatal, a 9 V transistor battery won’t kill you if you handle it in water. You can stick it on your tongue, to see if it’s dead (if you are in a hurry, don’t have a VOM, and don’t mind a little tingle).
I just think that situation with the poor teenager is very suspicious. At the least, if it is an equipment problem, it needs to be solved, and chargers recalled, before some one else is killed.