Today it's easy to get a TIN, as foreigners since 1st January 2024 are income taxable in Thailand, if the stay in the country for 180 days or more combined during a calendar year.
A British government pension might already be income taxed in Britain.
£12,000 a year is around 500,000 baht in 2024-currency exchance rate. If that is the amount transferred into Thailand and your English friend is retiree age – 60 years or older – he will on his Thai-income have a personal deduction of 60,000 baht; a retiree deduction of 190,000 baht; 50% income deduction up to 100,000 baht; and being tax exempt for the first 150,000 baht. This totals 500,000 baht, so there is likely no tax due from £12,000 previously transferred annually into Thailand.
The previous years the common practise was that if you had no tax to pay you didn't need to fill a tax return form. From income year 2024 many, if not most, tax offices wish all tax resident foreigners to fill in a tax return form, even they do not need to any income tax.