There is impact on dementia
by very high sugars in the range beyond 250. Her average sugar has to be maintained at around 180 mg/dl looking at her age, and physical state.
If she's given insulin
shots before each major meal then checking her pre-meal sugars will help in deciding the units in the shots. Once her post meal sugars fall upto 200 mg/dl then you do not need to monitor them very frequently. But any unusual symptom should warn people around her to check her sugars to avoid hypoglycemia
I hope you got your answer. get back for any further query.