Nothing in the article says the mother let the child out or even know the child got out.
I have raised four children some living near a pond Never did I have one get out at night. All my doors were alarmed so they if anyone went out in the middle of the night (or came in) I would know instantly. In addition I had Slide locks at the top of all doors and several windows. Windows were securely locked and not accessible to a five year old. Keeping a child safe takes forethought and care. I am sorry this mom learned that too late.