you must get them tested if they have Alzheimer's Disease etc. An Occupational Therapist can do MOCA test to score their cognition.
For temporizing measure, get them a white board and write to them stuff to remember.
Absolutely write on it permanently to: switch off the gas after cooking, not to open the door to anyone else unless known to them, to absolutely lock up the front door at night.
Train them (yes, I know its kinda harsh to write TRAIN) to look at the board everyday.
No supplements that can help in brain functions, mate.