I read the first couple of pages earlier, and then got distracted, so my point may well have been covered in the meantime.
Maybe the mum was doing the best, and at that point of time, safest thing by leaving the toddler in the car, safely strapped in, presumably locked?
I regularly leave my 15 month old in the car whie I take my older daughter into school. It is the safest place to leave him. He is not yet walking, and i cannot carry him as well as dd1's lunchbox, and keep hold of dd1 adequately (dd1 is 9, with severe ASD). If I took ds out too, then I would not be holding either child adequately, and so both would be less secure than if I leave him in the car and take dd1 in.
Sometimes he cries. I hate it that he does, but it is still safer to leave him there than bring him along (at times).
In the car, he is strapped in (and cannot undo his straps, and none of mine could undo their straps at 2 either). the doors are childlocked. the car is also locked, so risk of abduction is negligible. what on earth is actually going to happen to him?
I can understand why the OP might wait to check he is alright, and someone is coming back to him. But anything other than that is a step too far. If someone had spoken to me like that when dd2 was that age (so dd1 was about 3.5 - probably her and my lowest point in terms of behaviour and life being generally shit), I would have been absolutely distraught. It would have added a whole load of completely unneeded stress and judgement to a situation where I was only doing my best. Because that is all peopel can do.
The OP does not have the full facts, and neitehr did she even bother to let the other mother explain (not that she needed to). The OP had decided she was right, and that no circumstances ever would change her mind.
Newsflash: sometimes life is not that simple. Sometimes you can only do the best in any given situation, and there isn't actually a right answer, just a least-wrong one.