Hi, Thank you for your question and welcome. My name is AJ and I will assist you. A person can make a will and/or grant a power of attorney if they have dementia. However they will need their doctor and solicitor to confirm that at the time of writing the will and or giving the power of attorney they have the capacity to do so.
This is known as testamentary capacity.
Practically speaking if the diagnosis is at an early stage then you should go and speak to this person's doctor as soon as possible and find out what would be required (or even if it is possible) for the doctor to write an opinion that the person has the capacity to sign a will.
I look forward to hearing from you. Kind regards