I don't think that is what did it, simply because the wall wart is close to the wall with only a 5V lead into her phone. Wet all over, her body had a significantly reduced electrical resistance giving an easy path into her blood stream, which is largely salt water. I suspect water ran down her hand, down the cord, and into the connector. The 5V apparently did push up to .2 amperes into her nervous system, which is all it takes to kill someone.
Dry skin exhibits over 300,000 ohms of resistance. Wet skin, maybe a hundred.
I think she plugged her phone charger into the extension cord while she was in the tub, that will do it right there quick.
I don’t recall that math question being asked at DeVry.
You have a point but the 5 Volts you refer is a Direct Current source.
If the resistance between two locations on her body were 25 Ohms then 1 watt of DC power of heat is applied.
I am inclined to think there is more to this story and I don't think 5 VDC can harm the nervous system such that it doesn't recover.