My Mom is pretty smart and together, but over the last couple of years she just doesn’t seem to understand that food like milk or meat, cannot be left out on the counter. When I step in, respectfully to let her know, she gets upset and asks how come she has made it all these years then. I’m really perplexed, as normally she’s rational and she also only does this occasionally. Any ideas? I have dinner with her every night, so can stave most of this off...
If she's not able to recognize food safety, I'd bet there are other things that are risky that are going on too. I might stay with her for a couple of days so you can get a better idea of what they are.
There are some really good articles here on dementia. I suggest that you look them up.