No
Overview
There is no general rule requiring a seller to determine the mental condition of a buyer. However, if a seller knows or has good reason to know that a buyer does in fact had dementia or some other mental problem, and is not or may well not be able to understand the nature of the deal or the reasons why it is or is not a good bargain, if the seller took advantage of the buyer's mental condition, then the seller may be liable.
Also, if the buyer's condition is such that the buyer is deemed not to be capable of properly entering into a contract, that is the buyer lacks the capacity to contract, then the contract may well be void from teh4 start.
The Video
The video linked in the question shows a man confronting a salesman who, he alleges, sold two houshold doors to his mother for a total of just under 6,000 pounds UK. The son claims that it was obvious that his mother was impaired. He claims that the only problem was that one door had a broken handle and latch, which could easily be repaired for 50 pounds. He claims that the salesman falsely states that the doors were "not safe" without mentioning that the one with a problem could easily be repaired, and need not be replaced. He also claims that the salesman took checks in payment, in spite of the printed terms on the invoice saying that no payment was due until after installation. He says repeatedly that the salesman "pressured" his mother into making this purchase, and describes it as "robbery".
No proof is shown in the video, and no court decision is refereed to. I express no vieo as to whether the claims of the son in the video are accurate. But I do discuss situations where similar claims are accurate.
Consumer Protection from Unfair Trading Regulations 2008 (UTR)
It sees that a sales agent acting similarly to the one accused in the video might have committed unfair practices and thus offenses under the UTR, specifically "aggressive selling", "misleading statements", and "misleading omissions".
Under the UK Consumer Protection from Unfair Trading Regulations 2008 (UTR) regulation 3, specifically 3(3) and 3(4):
(3) A commercial practice is unfair if—
(a) it contravenes the requirements of professional diligence; and
(b) it materially distorts or is likely to materially distort the economic behaviour of the average consumer with regard to the product.
(4) A commercial practice is unfair if—
(a) it is a misleading action under the provisions of regulation 5;
(b) it is a misleading omission under the provisions of regulation 6;
(c) it is aggressive under the provisions of regulation 7; or
(d) it is listed in Schedule 1.
UTR regulation 5 provides in relevant part:
5.—(1) A commercial practice is a misleading action if it satisfies the conditions in either paragraph (2) or paragraph (3).
(2) A commercial practice satisfies the conditions of this paragraph—
(a)if it contains false information and is therefore untruthful in relation to any of the matters in paragraph (4) or if it or its overall presentation in any way deceives or is likely to deceive the average consumer in relation to any of the matters in that paragraph, even if the information is factually correct; and
(b) it causes or is likely to cause the average consumer to take a transactional decision he would not have taken otherwise
UTR Regulation 7 on Aggressive commercial practices provides in relevant part:
7.—(1) A commercial practice is aggressive if, in its factual context, taking account of all of its features and circumstances—
(a) it significantly impairs or is likely significantly to impair the average consumer's freedom of choice or conduct in relation to the product concerned through the use of harassment, coercion or undue influence; and
(b) it thereby causes or is likely to cause him to take a transactional decision he would not have taken otherwise.
(2) In determining whether a commercial practice uses harassment, coercion or undue influence account shall be taken of—
...
(c) the exploitation by the trader of any specific misfortune or circumstance of such gravity as to impair the consumer's judgment, of which the trader is aware, to influence the consumer's decision with regard to the product;
UTR regulation 8 provides in relevant part:
8.—(1) A trader is guilty of an offence if—
(a) he knowingly or recklessly engages in a commercial practice which contravenes the requirements of professional diligence under regulation 3(3)(a); and
(b) the practice materially distorts or is likely to materially distort the economic behaviour of the average consumer with regard to the product under regulation 3(3)(b).
UTR schedule 1(12) describes as unfair:
- Making a materially inaccurate claim concerning the nature and extent of the risk to the personal security of the consumer or his family if the consumer does not purchase the product.
UTR regulations 9 through 12 make unfair practices, as described in regulations 3 through 7 and schedule 1, offenses punishable by fine or imprisonment.
These might well apply to the practices alleged in the video linked in the question
Financial Conduct Authority
A Guidance from the UK Financial Conduct Authority is available online.
This seems to apply more to sales of financial products (e.g. investments) than physical goods such as doors. It is also a draft, not a final regulation. But it more specifically deals with people vulnerable due to metal issues, and might show the trend of regulation on such issues in the UK.
2.1 In our Approach to Consumers we define a vulnerable consumer as ‘someone who,
due to their personal circumstances, is especially susceptible to detriment, particularly
when a firm is not acting with appropriate levels of care’ (as presented in our Occasional
Paper 8 on Consumer Vulnerability)
...
2.14 (g) Exposure to mis-selling – vulnerable consumers may be more likely to fall victim to
pressure selling, or be provided with the wrong information about potential products or services by staff who do not understand their circumstances. People with mental health problems, for example, are more likely to have mistakenly bought a product on
credit and 3 in 4 of those felt pressured into doing so.
2.15 We expect the firms we regulate to treat customers, including vulnerable customers, fairly. Our previous work has shown that not all firms treat vulnerable consumers fairly, and those consumers face a significant risk of harm. Vulnerability is, therefore, a key priority for the FCA
Conclusion
I have not found any UK law or regulation that requires firms or people doing general business to determine if a potential customer suffers from Alzheimer's, dementia, or any similar mental issue. However, taking advantage of people who exhibit symptoms of such problems might be found to be an unfair practice, and so lead to civil or criminal liability