Can you die when your plugged-in mobile phone falls in the bathtub?
The phone itself can’t develop enough potential to cause a shock with burns - the battery is only 3.8V. Further, any stray currents would have been local to the phone, and would not have found a body path.

What might have happened is that she reached for the cord to plug in the phone, and she got shocked by the cord, and dropped the phone... and was not able to get the cord away. Very sad - what an unfortunate way to lose someone.

Then the underlying problem leading to this would be two things: a faulty charger, and a non-functioning or not-present GFCI that should have tripped.