Responding to Platty's query, not the OP.
The relevant term is 'testamentary capacity'. It's best to consult a solicitor in these circumstances. The solicitor may deem the person to have testamentary capacity but will usually arrange for an independent assessment by a psychiatrist. It is also possible that the solicitor may wish to record the discussion with the testator in case there is ever any question about what was said.
Solicitors are bound by Law Society professional codes of conduct to do their best to ensure that everything is done correctly. They risk losing their career if they get it wrong and could face a huge fine or even prison if they are criminally negligent. You can understand why they are very cautious in these circumstances.
Having said all that, the law presumes that an adult has capacity unless there is evidence to the contrary.
It is not 'illegal' for someone to create a new Will. However, the Will could later be challenged if that person has an existing AD diagnosis. It's best to get professional advice.
BTW, the current Will might still be OK. It really depends on what needs to be achieved. If the Will refers to property that the person no longer owns, then this provision will just fail when the Will is executed. If the executors are dead or otherwise cannot act, this can be amended via a legally executed codicil (amendment document). Or new executors can be appointed by the courts when the testator dies.
It is also possible to apply to the Court of Protection for a new Will if this is really necessary due to a change in circumstances of a person who lacks capacity.
Testamentary capacity in my own experience:
When my mum made her last Will, and her POAs, she made some unwise choices. Not about the provisions but about who she chose as Attorneys and Executors (unwise for practical reasons, not because the people themselves were unsuitable). I would say that her early stage dementia affected her judgment. That did not stop her from insisting on what she wanted, despite contrary advice from the solicitor. The solicitor later told me that she had talked to my mother on 3 separate occasions and was satisfied that she was clear about what she wanted.
The documents were finally signed after my mother had spent 4 months in hospital, very ill, and had obvious mental impairment. The solicitor told me that although my mother was by that time quite confused she still stated the same wishes and therefore the solicitor was professionally satisfied that my mother retained testamentary capacity, and capacity to sign her POA documents.
An attempt by a relative a couple of years later to change these documents failed because by that time my mother had been assessed by her psychiatrist as having lost capacity with regard to her financial affairs.