Leaving the engine running with a very young kid, especially toddler or younger, alone in the car is not good judgement, period, 10 feet away or not. Yes, that includes dipping back into the house for one or more forgotten items, as an easy example.
Do I think she’s a bad mom? No, not at all! But it was demonstrably a bad idea. The answer doesn’t change, if the father is the extant parent, so w/e on that.