That is what I am trying to change.
Considered and dismissed it? Okay, at least you considered it.
There is a big difference between taking a few minutes out within walking distance, and driving to the shops.
And, yes, actually I would consider reporting that, as she locked the door and left the property. I may not actually report it, but I would consider it.
The mum may genuinely believe she did nothing wrong. Or may have issues. Or may not have family support. Any of these are reason enough to consider intervention, as it will help.