How do you know she wasn't hanging laundry in the garden and a family member left the house and didn't shut the door properly? People always jump on the Mother (but never the Father though) without knowing the full facts.
Something similar happened on my street some years ago and it was my neighbour's idiot brother who forgot to shut the baby gate. The Mother was cooking a Sunday dinner at the time and had her door open because the kitchen gets hot. The child wasn't hurt (and didn't get that far from the house), but a few people had ago at her. Not one of them said anything to her brother who had left the gate open though even when he said it was his fault.
I was hanging laundry or I was cooking Sunday dinner or whatever. What excuse would let you feel OK that your kid was run over by an 18-wheeler? BOTH parents are solely responsible for the safety of their baby. PERIOD! NO EXCUSES!