Perhabs the usb cable was too short and the person used a mains extension cord, placing it close to the water?
In any case, everyone in the planet knows that electricity+water is a nasty combination. Don't even try.
Except this person, it seems. So that was Darwin doing its job, nothing to worry about.
I smell a lot of bs here, but this could be a combination of old/crappy electric installation + chinesium charger.
Humid zones (Talking about house places, control your dirty minds) have their own leakage detector (Again, talking about house places!), which use a more sensible rating.
In UE, while the "main" breaker uses 30mA rating, bathrooms use it's own with 5mA rating. So it's extremely hard to die electrocuted.
This didn't exist 30 years ago, so old installations might not be so lucky. I've even seen single-pole breakers opening the neutral instead the live! Yes, freaking dangerous!
So there're a lot of possible factors here. But in the 99.9% of the cases, this is impossible.