Something is really suspicious here. 30 milli amps, (0.030 amps, thirty thousandths of an Amp) when applied across the heart, is minimum lethal amperage. BUT YOU NEED ENOUGH VOLTAGE to penetrate the skin to overcome its resistance. Dry skin can resist 75 volts in ideal conditions, 30 Volts or less is considered relatively safe.
NO WAY IN HELL is 5 volts from a cell phone charger enough to penetrate skin and cause death. For example, take a 9 Volt battery and handle it in the bathtub. It is DC, just like the charger output, but almost double the voltage, and capable of more than 30 mA, nothing will happen.
The only way I can see it happening is if the charger was plugged into an extension cord and she grabbed the 120 Volt extension cord side of the charger (where it plugs in) with a dripping wet hand. Previous posters were right in recommending a GFCI (ground fault breaker). They are required in all wet locations.
GFCI work by measuring not only total amperage, i.e. 15 Amps or 20 Amps, but also measuring difference between what goes out hot side and what returns to the box on the neutral side. If there is more than 5 mA (0.005 Amps, 5 thousandths of an Amp) difference, like when some current is going to ground through you, instead of returning to the breaker box, they will immediately trip off.
That sounds likely to me too. In fact, aren't plugs in bathrooms usually placed farther away from the bathtub than the length of most razor and cell phone cords?
Thanks for the info re: GFCIs.
But the crux: Why is it bad to plug a power strip into another power strip? I do that in my garage when I need a few extra feet of cord.
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.