My four year old daughter wakes up before the wife and I do sometimes (thanks to our 3 month old). She lets herself out of her room, uses the bathroom, brushes her teeth, grabs a fruit for her "pre-breakfast" checks if anything good is on the Disney channel and if nothing is, she will get herself some water color paints/paper/2 cups of water and get to it. She's more capable than some adults I know.
I would never let her out of my damn sight if a pool was involved, though (despite being a rather proficient swimmer for her age).
Bad call on the mother's part and she will regret it for the rest of her life, no need to pile criminal charged on top of it as well.