"...after living in the USA for about 25 years. "
I missed this bit initially.
I presume that they have US citizenship (or at least a Green Card?) as well as British citizenship?
If that is the case then they will still be required to report their income in the UK every year to the IRS as long as they retain their US citizenship or Green Card.
This can be a real hassle to do. Americans have what is called the "Foreign Earned Income Exclusion". What this means is that, from 2024, the first US$126k (£100k) of income is free of any US tax - but still does need to be reported, which can be a real pain. There is also something called the "Foreign Tax Credit" which offsets any tax you pay in the UK, for example, against any tax you may owe the US government.
Basically, if a US citizen earns over US$126k and lives in a high tax country (like the UK) it's usually better to go for the Foreign Tax Credit if they have any income in the US and if they live in a low tax country then it's usually better to go for the Foreign Earned Income Exclusion. But there are exceptions to this.
If they earn less than US$126k then it doesn't matter but they still do need to file with the IRS a Form 1040 and Form 2555 for the Foreign Earned Income Exclusion.
America really does hate it when their people go overseas.