Hi, jknight. I'm relieved to hear that your mother isn't currently struggling with other issues, but I agree with you that the food/nutrition one is a worry.
Given her age and situation (sorry to hear about your father's passing), I can see that it is very likely she doesn't want to be bothered with cooking and preparing meals the way she used to. I think a lot of us don't always feel like cooking, especially just for one person!
So here are some thoughts I have:
-she may not be feeling hungry, either due to the dementia, possibly other medical issues, maybe medication, or just not having as much of an appetite anymore. This may make her less inclined to prepare a meal. This was a problem with my mother (for all the above reasons) and I've heard others here say this as well.
-the dementia may be a factor in this, but a subtle one. Many PWD (persons with dementia), at any stage of the disease but especially earlier on, will express that things "are too difficult" or that they "can't be bothered" or that they don't want to/are unable to muster up the energy to do things they used to do. One theory is that the disease robs the brain and body of energy; another is that it takes so much energy and effort to do what you used to do, or to cope, that you just don't have much left over. Whatever the cause, it's a common story here on TP and in my support groups.
-your guess that she is no longer seeing/reading/understanding sell by and use by dates, is a good one. If I had said to my mother, oh, look, the butter has expired, she would have replied, throw it out! But she never looked or saw or understood (the oldest butter I threw out from her fridge was four years expired, I kid you not). Some of it could be due to eyesight or lighting or print that's hard to see or other external factors, but some of it is clearly the dementia interfering.
I think my best suggestion right now, would be some combination of:
-company at mealtimes (family member, friend, carer)
-a lunch club or day care where there's a meal
-maybe a meal delivery? do you have Meals on Wheels in the UK? (please forgive this ignorant Yank) preferably combined with some company, as a reminder to eat
-try different sorts of food that may have more appeal--think nutritious snacks, rather than full meals
-talk to the GP and consider getting her checked for deficiencies, and adding any needed vitamin/mineral supplements
TP is a great place to come and say whatever is on your mind, ask questions, do a search and read old threads, or whatever you need. It's always open!